What happens when Apple Music ends?

Since Apple has not made public any "plans" for the end of AM, that would be pretty hard to discuss. But considering how well the various streaming services are doing, how they have cut into the sales of cd's and even digital downloads and how many paid subscribers AM has already, I don't think that is a major concern.

As far as theorizing - yes, you would lose any and all music you added/downloaded from AM to your iCloud Music Library - just as if you decided to cancel the subscription on your own. At that point you would have to purchase all the deleted albums/tracks that you wanted to keep having access to.

As far as whether it is "worth it" is a personal decision. I purchased - before the advent of AM - probably at least 20-25 full albums a year. So even at a discounted price of $10 each, that is more than double what the AM subscription costs me. And I have access to a lot of artists and albums that I would not have ordinarily added to my library. So for me - even if I have to go an purchase whatever albums I would have purchased down the road, the access to the service is worth it.

Apple Music is a subscription service which grants you access to the entirety of the iTunes store catalog to listen to.

You do not in any way own the music you get from this service, but are merely licensed to listen to it as long as you pay for the subscription.

Should the service end at any time, the music you may have added to your library through this service would most likely become unplayable just as if you had cancelled the service yourself.

If you wish to maintain access to the music you must buy it outright either from the iTunes store or as a physical CD.

This is no different than if say Netflix were to stop their service. You do not own the movies on Netflix, you merely license them to watch them. Apple Music just adds the benefit of being able to download them to your device so you can play them without an internet connection.

Almost correct. The library that Apple Music is not the entire iTunes Store library - only the songs and albums that the artists and their labels allow to be streamed. There is much more music in the store library.

Correct, there are some songs that are not streamable. but its still quite a large part of the catalog. So "entirety" is not exactly accurate.