Of course it's whack a mole. Moat things in infosec are, that doesn't mean browsers shouldn't ship with secure defaults or present trustworthy info in the address bar.
This game off whack-a-mole feels different. Unlike the typical "memory corruption of the week", this kind of stuff isn't fixed by a simple browser update and inherited "for free" by all sites. And, this kind of fix doesn't enable a browser to ship with a secure default. Instead, it adds a new thing you have to opt into and retroactively add to all existing links on your site. That is a fair bit of work, and adding more and more of those kinds of features for nominal gain is a tough sell. That is a much more painful game of whack-a-mole and isn't an approach that scales well. The CSP solution at least has a potentially simpler scaling solution to the problem.
Unless we are talking about something terribly dire (arbitrary code execution) browser vendors are super unlikely to change behavior that has existed, and potentially relied upon, for many years. The bar for changing existing behavior is extremely high and this kind of attack won't come anywhere near meeting it. So, the only realistic solution is something that a site opts into (or out of depending on your perspective). CSP would at least let the site that is a potential victim protect itself. And, if there was a good reason to let a partner site have window ref (I could imagine something related to payment providers and modal pop up payment flows), they could opt in to that. It would look something like.
window-ref 'self' PayPal.com
Something like that would let the site reference their own windows as well as grant access to a "trusted partner" like PayPal.
Browser vendors do 'phase out' old behavior and phase in new ones. I understand "don't break the web", but as someone else famously replied, "the web is a self healing mechanism". Look at what browsers have done re: forms submitted over HTTP.
A maintained site that relies on window.opener should, after a 24 month period of angry console warnings saying a change needs to be made, actually make that change.