diff --git a/Mathlib/LinearAlgebra/Dimension/Finite.lean b/Mathlib/LinearAlgebra/Dimension/Finite.lean
index 0944ed66c2a313..6b161ad48fb70a 100644
--- a/Mathlib/LinearAlgebra/Dimension/Finite.lean
+++ b/Mathlib/LinearAlgebra/Dimension/Finite.lean
@@ -196,8 +196,9 @@ lemma exists_set_linearIndependent_of_lt_rank {n : Cardinal} (hn : n < Module.ra
 lemma exists_finset_linearIndependent_of_le_rank {n : ℕ} (hn : n ≤ Module.rank R M) :
     ∃ s : Finset M, s.card = n ∧ LinearIndepOn R id (s : Set M) := by
   rcases hn.eq_or_lt with h | h
-  · obtain ⟨⟨s, hs⟩, hs'⟩ := Cardinal.exists_eq_natCast_of_iSup_eq _
-      (Cardinal.bddAbove_range _) _ (h.trans (Module.rank_def R M)).symm
+  · obtain ⟨⟨s, hs⟩, hs'⟩ := exists_eq_ciSup_of_not_isSuccLimit
+      (Cardinal.bddAbove_range _) (h.trans (Module.rank_def R M) ▸ not_isSuccLimit_natCast n)
+    rw [← Module.rank_def, ← h] at hs'
     have : Finite s := lt_aleph0_iff_finite.mp (hs' ▸ natCast_lt_aleph0)
     cases nonempty_fintype s
     refine ⟨s.toFinset, by simpa using hs', by simpa⟩
diff --git a/Mathlib/Order/SuccPred/CompleteLinearOrder.lean b/Mathlib/Order/SuccPred/CompleteLinearOrder.lean
index d514475c7b9f9a..d33bcb679fd3c0 100644
--- a/Mathlib/Order/SuccPred/CompleteLinearOrder.lean
+++ b/Mathlib/Order/SuccPred/CompleteLinearOrder.lean
@@ -87,17 +87,32 @@ section ConditionallyCompleteLinearOrderBot
 variable [ConditionallyCompleteLinearOrderBot α] {f : ι → α} {s : Set α} {x : α}
 
 /-- See `csSup_mem_of_not_isSuccPrelimit` for the `ConditionallyCompleteLinearOrder` version. -/
-lemma csSup_mem_of_not_isSuccPrelimit'
-    (hbdd : BddAbove s) (hlim : ¬ IsSuccPrelimit (sSup s)) : sSup s ∈ s := by
+lemma csSup_mem_of_not_isSuccPrelimit' (hlim : ¬ IsSuccPrelimit (sSup s)) : sSup s ∈ s := by
   obtain rfl | hs := s.eq_empty_or_nonempty
   · simp [isSuccPrelimit_bot] at hlim
-  · exact csSup_mem_of_not_isSuccPrelimit hs hbdd hlim
+  · apply csSup_mem_of_not_isSuccPrelimit hs _ hlim
+    contrapose! hlim
+    rw [csSup_of_not_bddAbove hlim, csSup_empty]
+    exact isSuccPrelimit_bot
 
 /-- See `exists_eq_ciSup_of_not_isSuccPrelimit` for the
 `ConditionallyCompleteLinearOrder` version. -/
-lemma exists_eq_ciSup_of_not_isSuccPrelimit'
-    (hf : BddAbove (range f)) (hf' : ¬ IsSuccPrelimit (⨆ i, f i)) : ∃ i, f i = ⨆ i, f i :=
-  csSup_mem_of_not_isSuccPrelimit' hf hf'
+lemma exists_eq_ciSup_of_not_isSuccPrelimit' (hf' : ¬ IsSuccPrelimit (⨆ i, f i)) :
+    ∃ i, f i = ⨆ i, f i :=
+  csSup_mem_of_not_isSuccPrelimit' hf'
+
+lemma csSup_mem_of_not_isSuccLimit (hne : s.Nonempty) (hbbd : BddAbove s)
+    (hlim : ¬ IsSuccLimit (sSup s)) : sSup s ∈ s := by
+  rw [isSuccLimit_iff_of_orderBot, not_and_or, not_ne_iff] at hlim
+  refine hlim.elim (fun h ↦ ?_) csSup_mem_of_not_isSuccPrelimit'
+  obtain ⟨a, ha⟩ := hne
+  obtain rfl | ha' := eq_or_ne ⊥ a
+  · rwa [h]
+  · exact (h ▸ ha'.bot_lt'.trans_le <| le_csSup hbbd ha).false.elim
+
+lemma exists_eq_ciSup_of_not_isSuccLimit [Nonempty ι] (hbbd : BddAbove (range f))
+    (hf : ¬ IsSuccLimit (⨆ i, f i)) : ∃ i, f i = ⨆ i, f i :=
+  csSup_mem_of_not_isSuccLimit (Set.range_nonempty _) hbbd hf
 
 theorem Order.IsSuccPrelimit.sSup_Iio (h : IsSuccPrelimit x) : sSup (Iio x) = x := by
   obtain rfl | hx := eq_bot_or_bot_lt x
@@ -117,7 +132,7 @@ theorem sSup_Iio_eq_self_iff_isSuccPrelimit : sSup (Iio x) = x ↔ IsSuccPrelimi
   refine ⟨fun h ↦ ?_, IsSuccPrelimit.sSup_Iio⟩
   by_contra hx
   rw [← h] at hx
-  simpa [h] using csSup_mem_of_not_isSuccPrelimit' bddAbove_Iio hx
+  simpa [h] using csSup_mem_of_not_isSuccPrelimit' hx
 
 theorem iSup_Iio_eq_self_iff_isSuccPrelimit : ⨆ a : Iio x, a.1 = x ↔ IsSuccPrelimit x := by
   rw [← sSup_eq_iSup', sSup_Iio_eq_self_iff_isSuccPrelimit]
diff --git a/Mathlib/SetTheory/Cardinal/Arithmetic.lean b/Mathlib/SetTheory/Cardinal/Arithmetic.lean
index eece428bbd2cb0..ee8638574538e3 100644
--- a/Mathlib/SetTheory/Cardinal/Arithmetic.lean
+++ b/Mathlib/SetTheory/Cardinal/Arithmetic.lean
@@ -355,8 +355,7 @@ protected theorem ciSup_add (hf : BddAbove (range f)) (c : Cardinal.{v}) :
   refine le_antisymm ?_ (ciSup_le' this)
   have bdd : BddAbove (range (f · + c)) := ⟨_, forall_mem_range.mpr this⟩
   obtain hs | hs := lt_or_ge (⨆ i, f i) ℵ₀
-  · obtain ⟨i, hi⟩ := exists_eq_of_iSup_eq_of_not_isSuccLimit
-      f hf (not_isSuccLimit_of_lt_aleph0 hs) rfl
+  · obtain ⟨i, hi⟩ := exists_eq_ciSup_of_not_isSuccLimit hf (not_isSuccLimit_of_lt_aleph0 hs)
     exact hi ▸ le_ciSup bdd i
   rw [add_eq_max hs, max_le_iff]
   exact ⟨ciSup_mono bdd fun i ↦ self_le_add_right _ c,
@@ -384,8 +383,7 @@ protected theorem ciSup_mul (c : Cardinal.{v}) : (⨆ i, f i) * c = ⨆ i, f i *
   refine le_antisymm ?_ (ciSup_le' this)
   have bdd : BddAbove (range (f · * c)) := ⟨_, forall_mem_range.mpr this⟩
   obtain hs | hs := lt_or_ge (⨆ i, f i) ℵ₀
-  · obtain ⟨i, hi⟩ := exists_eq_of_iSup_eq_of_not_isSuccLimit
-      f hf (not_isSuccLimit_of_lt_aleph0 hs) rfl
+  · obtain ⟨i, hi⟩ := exists_eq_ciSup_of_not_isSuccLimit hf (not_isSuccLimit_of_lt_aleph0 hs)
     exact hi ▸ le_ciSup bdd i
   rw [mul_eq_max_of_aleph0_le_left hs h0, max_le_iff]
   obtain ⟨i, hi⟩ := exists_lt_of_lt_ciSup' (one_lt_aleph0.trans_le hs)
diff --git a/Mathlib/SetTheory/Cardinal/Basic.lean b/Mathlib/SetTheory/Cardinal/Basic.lean
index b60366d643c303..472f88200824f5 100644
--- a/Mathlib/SetTheory/Cardinal/Basic.lean
+++ b/Mathlib/SetTheory/Cardinal/Basic.lean
@@ -399,9 +399,11 @@ theorem isStrongLimit_aleph0 : IsStrongLimit ℵ₀ := by
 theorem IsStrongLimit.aleph0_le {c} (H : IsStrongLimit c) : ℵ₀ ≤ c :=
   aleph0_le_of_isSuccLimit H.isSuccLimit
 
+@[deprecated exists_eq_ciSup_of_not_isSuccLimit (since := "2026-04-13")]
 lemma exists_eq_natCast_of_iSup_eq {ι : Type u} [Nonempty ι] (f : ι → Cardinal.{v})
-    (hf : BddAbove (range f)) (n : ℕ) (h : ⨆ i, f i = n) : ∃ i, f i = n :=
-  exists_eq_of_iSup_eq_of_not_isSuccLimit.{u, v} f hf (not_isSuccLimit_natCast n) h
+    (hf : BddAbove (range f)) (n : ℕ) (h : ⨆ i, f i = n) : ∃ i, f i = n := by
+  rw [← h]
+  exact exists_eq_ciSup_of_not_isSuccLimit hf (h ▸ not_isSuccLimit_natCast n)
 
 @[simp]
 theorem range_natCast : range ((↑) : ℕ → Cardinal) = Iio ℵ₀ :=
diff --git a/Mathlib/SetTheory/Cardinal/Order.lean b/Mathlib/SetTheory/Cardinal/Order.lean
index 4c53f247b3c7b2..dcb15f24bf6de4 100644
--- a/Mathlib/SetTheory/Cardinal/Order.lean
+++ b/Mathlib/SetTheory/Cardinal/Order.lean
@@ -544,28 +544,21 @@ theorem exists_wellFoundedLT : ∃ (_ : LinearOrder α), WellFoundedLT α := by
 
 namespace Cardinal
 
-/-! ### Bounds on suprema -/
-
+@[deprecated exists_eq_ciSup_of_not_isSuccPrelimit' (since := "2026-04-13")]
 lemma exists_eq_of_iSup_eq_of_not_isSuccPrelimit
     {ι : Type u} (f : ι → Cardinal.{v}) (ω : Cardinal.{v})
     (hω : ¬ IsSuccPrelimit ω)
     (h : ⨆ i : ι, f i = ω) : ∃ i, f i = ω := by
   subst h
-  suffices BddAbove (range f) from (isLUB_csSup' this).mem_of_not_isSuccPrelimit hω
-  contrapose hω with hf
-  rw [iSup, csSup_of_not_bddAbove hf, csSup_empty]
-  exact isSuccPrelimit_bot
+  exact exists_eq_ciSup_of_not_isSuccPrelimit' hω
 
+@[deprecated exists_eq_ciSup_of_not_isSuccLimit (since := "2026-04-13")]
 lemma exists_eq_of_iSup_eq_of_not_isSuccLimit
     {ι : Type u} [hι : Nonempty ι] (f : ι → Cardinal.{v}) (hf : BddAbove (range f))
     {c : Cardinal.{v}} (hc : ¬ IsSuccLimit c)
     (h : ⨆ i, f i = c) : ∃ i, f i = c := by
-  rw [Cardinal.isSuccLimit_iff] at hc
-  refine (not_and_or.mp hc).elim (fun e ↦ ⟨hι.some, ?_⟩)
-    (Cardinal.exists_eq_of_iSup_eq_of_not_isSuccPrelimit.{u, v} f c · h)
-  cases not_not.mp e
-  rw [← nonpos_iff_eq_zero] at h ⊢
-  exact (le_ciSup hf _).trans h
+  subst h
+  exact exists_eq_ciSup_of_not_isSuccLimit hf hc
 
 /-! ### Indexed cardinal `prod` -/