You could still get the actual comics anywhere, convert em to .cbz, and chuck em in some folders named by series. In that sense, Tachiyomi should keep working forever.
It's really only the streamlined online sources that'll break over time, and no new features will get added.
Lucky for us, looks like it'll be getting forked and live on as "Mihon".
So does Tachiyomi also provide the external sources? My impression was they just did the app, and you got things from elsewhere, but I've never used it (or even heard of it before today).
It's also already got 3 forks, they link them on their website.
Tachiyomi is just a comic reader, but it can aggregate from A LOT of sources, both local files and websites. Each online source (mangadex, tapas) requires a plug-in to work. Those plugins need maintenance and workarounds for stuff like cloudflare blocks.
Needless to say a lot of content hosts aren't fans of a client that pulls content without the user ever seeing an ad or something, so these plug-ins tend to break.
There's a number of existing forks already. Most of them for porn. For the ones that have integrated with the last Tachiyomi update that removed native extensions, what you'll be able to do is set a repository as an upstream apk source for whatever extensions you want. There's plenty of those already on github. You just have to look for them.
I still have apps that I made to work on android 6, working just fine. Android has extremely good backwards compatibility.
The reason tachiyomis plug-ins would stop working is that the sources they pull content from keep blocking them, and there wouldn't be anyone to come up with new workarounds.
Android itself hasn't changed that much unless your app is overly reliant on old app permissions. Which tachiyomi isn't.