Right? My Pixel 4a still works like new but Google's dropping support so I have to get a new one or run a custom ROM. And the new phones don't have headphones jacks!
All that said, I mostly use Bluetooth headphones anyway now, and it's rumored that Google will switch to a 7 year support cycle, so I might just grab a Pixel 8 on Black Friday.
Google Play system updates are a thing since the first Pixel, and still continues to be updated to this day. So there still is some form of system updates.
Samsung S7 (same year as the first Pixel) when it reached end-of-service had to have a few security updates due to some pretty big android exploits. Pixel however did not need a security update because Play system took care of it. So even though your Pixel reached EoS, it actually didn't really.
Because modern apps may not support old OSs. And because security, I'm not sure if Google will still release security updates, but security is important.
Luckily it's much better now. My 6a has a 5 year support cycle, so I'll definitely get bored before the support ends. Also like you said, the 7 year support rumor too, which I think would be really good. Don't think they should be supported longer then that.
I've been reading about LineageOS for my 4a. Looks like it comes with a lot of caveats, and certain Google Apps not working. As much as I hate it, I do actually need some of that, I'm kinda stuck on stock android for now. Probably gonna grab the 8 or 7a on Black Friday.
assuming it uses MicroG, yeah it still lacks a lot of compatibility. I am using GrapheneOS and it has sandboxed google play, which basically gaslights the google play services into thinking it has full device access, but limits it to just a regular app that can be installed/uninstall at will. It's pretty cool and is like 99.9% compatibility with regular stuff which is cool.