Because the software is open-source - people could fork it before the change. It’s out there already. People can totally make their own little variants of Lemmy with added features, if that’s something they wanted to do. You can modify the code yourself and then self-host the modified version. No matter what Lemmy’s devs do… they have no power on your instance. A fork means you own the code.

People are already doing so, right now. AFAIK Lemmy by default doesn’t have the ability to disable downvoting, yet Beehaw and the instance I’m on (among others, probably) do have downvoting disabled.