I would not hold your breath for this. The holdover MP3 collector is such a small minority that no mainstream company is going to consider them from here on out.
> is there any other free service where I could upload my MP3 collection to
If there was, why would you waste your time? Either it's not going to be free or it's going to disappear.
GPM was nice in that it merged the cloud music with your own collection, but I'm pretty resigned to the fact that I'm going to lose that functionality when they finally shut it down.
For Spotify, Google, Amazon, Apple, etc. that's not a problem.
Are people going to cancel their accounts because they can't listen to a very small subset of music? Probably but it's going to be a tiny minority of users that's inconsequential to their bottom line.
There had been murmurs for a long time of Google Play Music going away, so I moved off ASAP. Looks like that was a good call.
Because voting with your wallet works.