>> 1. if we do find ourselves in a situation where changing this would break lots of users, will we consider ourselves beholden to them?
I think that it would be useful for Numpy's continued evolution to develop the ability to include code on a provisional basis. Other projects do this and they just have big bold "Experimental" notes everywhere that a user might go to learn about the functionality (docs, docstrings, blogposts). Some users will definitely get burned, yes, but the alternative is to burn all other users a little bit by moving too slowly and not trying things out.
This is different from how Numpy has operated in the past, but that might be ok.
>> 2. is it plausible that we'll find ourselves in that situation?
> Personally, for as long as this protocol is experimental, I’ll add a warning in the docs of sparse as well; saying this might disappear anytime
Yup. Same for Dask. We're pretty accustomed to this.