> There were also quite a few really old and niche records on there which possibly couldn't be put on streaming services due to the ownership of rights being unknown.

Music licensing (in the US at least) is actually pretty nice for this (from the licensee perspective anyway). There are mechanical licenses which allow you to use music for many uses without contracting with the rightsholders and clearinghouses whose job is to determine where to send royalties. So you can use the music and send reporting and royalties to the clearing houses and you're done.

Of course, you may want to contract with the rightsholders if you don't like the terms of the mechanical license; maybe it costs too much, etc. If you're Spotify or similar and you have specific contracts for most of the music, and have to pay mechanical license rates for the tail, it might make sense to do so in order to boast of a larger catalog.