Even if they don't brick them explicitly they will no longer provide security updates for them.

I'm on the same boat, smart TV has never been online, all content is just cast from media server/phone/tablet straight to chromecast. It works, no fuss, glitch free, and of course they will kill it.

My og chromecast frequently crash (I suspect due to running out of ram when casting contents) so I ended up replacing it with the chromecast ultra. The og chromecast is still usable if you can tolerate the crashing, but I imagine it's going to get worse as developers keep updating their cast players (basically a webpage that plays video) with new features that consume more and more ram.