I fully understand your concerns, but still I believe that you aren't asking yourself the right question. Why do you need to port newer Android versions on older devices at all? It seems like the real problem to me.

Why do running newer software must depend on a newer operating system? Do you believe that everything that is part of the android codebase is necessary to run a chat app?

Part of the problem is that we seemingly gave up on trying to make software stable, and now we are wrongly putting the blame on hardware manufacturers for not wanting to spend their whole life supporting it