They should do it. Old phones can be repurposed for stuff like smart mirrors, not having to worry about the battery in that case would be great.
Nowadays, pretty much every phone supports battery idle mode anyways, which is the same thing: They don't trickle charge the battery, they just run off the charger entirely.
If we get replace batteries, implementing this too is a negligible amount of effort.
I don't think that many phones support the idle mode I think Sony has one, Samsung only has a battery bypass when you're gaming. I think it's still a pretty rare feature but if you know something I don't please share