Something like an Atari 2600 (I have two, including a heavy sixer) is different from an XP box, imo. I'd not expect a 2600 to play Mame games. It's old, and collectible, but it's also very much out of date. No-one is trying to make a 2600 run 32-bit code.
Besides - no-one is saying you have to throw anything away. You can keep it, frame it for the wall, or mount it in a display case. You just might not be able to use it in a new release of Mame. A Mame compatible controller is very cheap, after all.
I'm aware that Mame now includes MESS. A confession: I've never been able to get MESS to work. Seriously. I still use Stella, and another Atari 400/800 emulator. I have also downloaded .171 of Mame, but for gaming purposes use .160, since it does everything I need.
The trouble with old code is regression testing, I think. Even if the code is old and unchanged, doesn't mean it'll still work in a new build. Dev's can talk about this in more detail, but I don't believe it's true to suggest that just because a piece of code has not been changed, and was working in the last version, that it will necessarily work in the new.