Teaching.

I had this problem with an overzealous junior developer and the solution was showing some different perspectives. For example John Ousterhout's A Philosophy of Software Design.

I tried this but they just come back with retorts like "OK boomer" which tends to make the situation even worse.

How do you respond to that?

The sibling comment says "fire them". That sounds glib, but it's the correct solution here.

From what you've described, you have a coworker who is not open to learning and considering alternative solutions. They are not able to defend their approach, and are instead dismissive (and using an ageist joke to do it). This is toxic to a collaborative work environment.

I give some leeway to assholes who can justify their reasoning. Assholes who just want their way because it's their way aren't worth it and won't make your product better.

Or, perhaps better, just let that hang for a moment - long enough to become uncomfortable - and then say "Try again."

As others have said, if they can't or won't get that that's unacceptable behavior, fire them. (jerf is more patient than I am...)

This is a discriminatory statement and it should be taken seriously.

Fire them.

To be honest, at the point where they are being insulting I also agree firing them is a very viable alternative.

However, to answer the question more generally, I've had some success first acknowledging that I agree the situation is suboptimal, and giving some of the reasons. These reasons vary; we were strapped for time, we simply didn't know better yet, we had this and that specific problem to deal with, sometimes it's just straight up "yeah I inherited that code and would never have done that", honestly.

I then indicate my willingness to spend some time fixing the issues, but make it clear that there isn't going to be a Big Bang rewriting session, but that we're going to do it incrementally, with the system working the whole time, and they need to conceive of it that way. (Unless the situation is in the rare situation where a rewrite is needed.) This tends to limit the blast radius of any specific suggestion.

Also, as a senior engineer, I do not 100% prioritize "fixing every single problem in exactly the way I'd do it". I will selectively let certain types of bad code through so that the engineer can have experience of it. I may not let true architecture astronautics through, but as long as it is not entirely unreasonable I will let a bit more architecture than perhaps I would have used through. I think it's a common fallacy of code review to think that the purpose of code review is to get the code to be exactly as "I" would have written it, but that's not really it.

Many people, when they see this degree of flexibility, and that you are not riding to the defense of every coding decision made in the past, and are willing to take reasonable risks to upgrade things, will calm down and start working with you. (This is also one of the subtle reasons automated tests are super super important; it is far better for them to start their refactoring and have the automated tests explain the difficulties of the local landscape to them than a developer just blathering.)

There will be a set that do not. Ultimately, that's a time to admit the hire was a mistake and rectify it appropriately. I don't believe in the 10x developer, but not for the usual egalitarian reasons... for me the problem is I firmly, firmly believe in the existence of the net-negative developer, and when you have those the entire 10x question disappears. Net negative is not a permanent stamp, the developer has the opportunity to work their way out of it, and arguably, we all start there both as a new developer and whenever we start a new job/position, so let me sooth the egalitarian impulse by saying this is a description of someone at a point in time, not a permanent label to be applied to anyone. Nevertheless, someone who insists on massive changes, who deploys morale-sapping insults to get their way, whose ego is tied up in some specific stack that you're not using and basically insists either that we drop everything and rewrite now "or else", who one way or another refuses to leave "net negative" status... well, it's time to take them up on the "or else". I've exaggerated here to paint the picture clearly in prose, but, then again, of the hundreds of developers I've interacted with to some degree at some point, there's a couple that match every phrase I gave, so it's not like they don't exist at all either.

You mean they literally say "ok boomer"? If so they are not mature enough for the job. That phrase is equivalent to "fuck off" with some ageism slapped on top and is totally unacceptable for a workplace.