I know I should probably raise this on the Github issue, and I'm posting this without having even looked at MPV yet to see if this is how it's already done or if there's any issues with this approach, but why not take the approach of 'profiles'? Ship with (at least) 2 profiles baked in - 'MPV legacy' and 'MPC-style'. Add more as desired. That way those who simply want to jump to a familiar system can choose MPC-style, those who want to stick with what they're familiar with from using MPV can, and the rest can customise as desired.
Any gradual changes just mean gradual changes to a keybind file. Anyone who doesn't like them can simply use their own keybind file from previous versions, or from their own modification.
Though rather than being baked in, these bindings have to be manually copied over the user's input.conf. mpv's config profiles don't extend to input.conf at the moment.
Any gradual changes just mean gradual changes to a keybind file. Anyone who doesn't like them can simply use their own keybind file from previous versions, or from their own modification.