Personally, and I know I may be in the minority here, but having gaps in my playlists, or missing albums, which actually stops me from listening to my music, is far more frustrating than allowing Roon to make an educated guess. I normally sit down to listen to music, not a particular format or version.
I’d be happy for Roon to pick the ’next best version’ (based on some mix of popularity & quality, maybe local over streaming, possibly adjustable as a preference), until such time as I could manually select a better version myself.
note: I am not talking about Roon automatically replacing an album track with a modern remix of it or replacing a recording of a classical piece with a different version performed by the same orchestra in a different time and place. Rather the ‘clear cut’ cases where the replacement version has the same key metadata, track count and length as the missing version.
And yes, I do appreciate that what might seem simple on paper can often be fraught with complexity and trade-offs which won’t make everyone happy, but that shouldn’t be a reason for not doing anything — anyway, sounds like Roon are already investigating a solution so fingers crossed 
While here, there’s a slightly different use-case (to tracks dropping out of Qobuz/Tidal) which is the ability to manually select an alternative version in a playlist which itself feels tangentially related to both this issue and the library/non-library conundrum.