A competent team would take it seriously. That said, you can be naive about it all too and not realize how serious licensing is to a company. So in my opinion it must be a company directive, and part of induction to make sure your developers understand the issue properly.

legal doesn't know what it doesn't know

implementing controls to prevent or detect this kind of copypasta is very hard.. even more so at larger "serious" companies that have dozens of development teams spread across the world working on an enormous catalog of products

this happens far more than people want to admit.. there's a very slippery slope when it comes to searching for help online.. it's amazing how quickly a google search leads to a stackoverflow post that has sample/example code which then winds up in your company's codebase.. even at "serious" places

writing a clear external code usage policy that's covered in those yearly training programs that developers hate and blow through as fast as possible is about as good you're going to get stopping this kind of thing

It's different to copy/paste code from SO and to copy/paste code from an open-source project with a non-permissive license and remove/ignore the license.

The controls to prevent developers from doing this sort of stuff in a serious company should be to let them know they will face disciplinary action/lose their job if they pull it.

Plenty of companies out there that blatantly violate the GPL.

What company? Tron? I do not believe it really exists, or if they do, they are serious.

There does not exist a license permissible enough that allow you to remove the license and replace it.

Three are a few licenses that allow the copyright reassignement target to do exactly that. This includes sometimes a termination clause.

They are rarely used.

WTFPL

Would you feel confident that a person could take a WTFPL licensed software, replace the license with GPL under their own name, and then go out and enforce the license?

Doing WTF you want is still limited by copyright law. You could argue that WTFPL is also an implied copyright reassignment, but I would not bet on it.

Public domain or CC0?

Public domain is special since its a natural aspect of copyright law rather than a license, where a person don't need permission once a work is in public domain.

CC0 tries to mimic this but here the tricky part of permissions comes into place. If I give you permission to do anything and you remove the permission text and claim the work as your own, do you still have permission? It a rather popular discussion in regard to cc0, and the consensus where I have seen it seems to be that you need the license in order to have permission. I have also seen lawyers arguing that copyright law itself has limits on what rights a license can give, such as attribution.