About multithreading, all kinds of redefinition have an impact. If I redefine a widely-used, low-level function, with hundreds of call sites – will each thread immediately call the new one without the bug, or will some still call the old one? Again, imposing a proper order would require protecting each function call with a lock, which is even worse for performance than protecting each slot access.

Still, we consider function redefinition a key feature of Common Lisp. So, redefinition of classes is in accordance with the spirit of the language.

Anecdotally, implementations that don't allow to redefine what Common Lisp doesn't mandate (e.g., in ABCL you cannot really redefine packages), in certain situations are painful to use, as they force you to delete & recreate everything, possibly even quitting Lisp and restarting.