diff --git a/packages/base/src/widget.ts b/packages/base/src/widget.ts
index 6dfd330717..f5cc378cc0 100644
--- a/packages/base/src/widget.ts
+++ b/packages/base/src/widget.ts
@@ -37,13 +37,6 @@ import {
  */
 const IPY_MODEL_ = 'IPY_MODEL_';
 
-/**
- * A best-effort method for performing deep copies.
- */
-const deepcopyJSON = (x: JSONValue) => JSON.parse(JSON.stringify(x));
-
-const deepcopy = (globalThis as any).structuredClone || deepcopyJSON;
-
 /**
  * Replace model ids with models recursively.
  */
@@ -538,18 +531,11 @@ class WidgetModel extends Backbone.Model {
         const serializers = (this.constructor as typeof WidgetModel).serializers || {};
         for (const k of Object.keys(state)) {
             try {
-                const keySerializers : any = serializers[k] || null;
-                let serialize = keySerializers?.serialize || null;
-                if (serialize == null && keySerializers?.deserialize === unpack_models) {
-                    // handle https://github.com/jupyter-widgets/ipywidgets/issues/3735
-                    serialize = deepcopyJSON;
-                }
-
-                if (serialize) {
-                    state[k] = serialize(state[k], this);
+                if (serializers[k] && serializers[k].serialize) {
+                    state[k] = serializers[k].serialize!(state[k], this);
                 } else {
                     // the default serializer just deep-copies the object
-                    state[k] = deepcopy(state[k]);
+                    state[k] = JSON.parse(JSON.stringify(state[k]));
                 }
 
                 if (state[k] && state[k].toJSON) {