In my opinion, if the install fails then this is a bug and should be fixed and should have no bearing on whether we would agree to allow this or not. I tend to agree with @kyrofa that I don’t see much of a distinction between a snap ceasing to provide an interface and choosing to auto-connect it or not - mpris is a dbus interface with a known API and if a snap is able to slot a this interface in the first place without any guarantee that it actually provides this interface then I don’t see why would would try and worry about that for declaring auto-connect for a snap which wishes to plug it.
If we do care about this, then perhaps the onus should be on the store to try and ensure any snap which slots such an interface actually does provide it… otherwise I don’t see there is much point worrying about it.
As such, +1 from me to auto-connect lyricfier:mpris to spotify:spotify-mpris