object freeze does not freeze the object when it is assigned to a new object

I have 2 questions here:-

1. Why obj2 = 5; is not throwing the error?

2. If I wish to throw during assignment obj2 = 5; what should I do? Considering obj2 is var or let.

"use strict"

function deepFreeze(object) {
    // Retrieve the property names defined on object
    const propNames = Object.getOwnPropertyNames(object);
  
    // Freeze properties before freezing self
  
    for (const name of propNames) {
      const value = object[name];
  
      if (value && typeof value === "object") {
        deepFreeze(value);
      }
    }
  
    return Object.freeze(object);
  }
  
  let obj2 = {
    internal: {
      a: null
    }
  };
  
  deepFreeze(obj2);
  
//   obj2.internal.a = 'anotherValue'; // fails silently in non-strict mode
//   obj2.internal.a; // null

obj2 = 5; // I want to throw error in this case but it is not throwing error.