Your questions are making the assumption that this would cost more and limit choice. This is not how markets work, this is exactly what government regulation and crony capitalism does.
Even in the case of your Spotify but not YT music example, what if there was a 5/month offer for 15mb internet with spotify streaming. Because spotify and your provider made a deal that subsidized the plan? At the end of the day, partnerships are not bad for the consumer. YT music could make a competing for deal for less money with more Google offerings. Also, this wouldn't eliminate an unlimited option across the board. When has competition made something more expensive? Its regulations like NN which pour amber over a system and make it impossible to be cheaper or better.
Even in the case of your Spotify but not YT music example, what if there was a 5/month offer for 15mb internet with spotify streaming. Because spotify and your provider made a deal that subsidized the plan? At the end of the day, partnerships are not bad for the consumer. YT music could make a competing for deal for less money with more Google offerings. Also, this wouldn't eliminate an unlimited option across the board. When has competition made something more expensive? Its regulations like NN which pour amber over a system and make it impossible to be cheaper or better.