Another point of view: there won't be a lot of people in the world being able to play guitar in a concert after 17 hours of training. Additionally, a clever player will do the guitar lesson together with (before) the rehearsal (needing 15) or/and the vocal lessons (needing 20). I see no big harm.
You are wrong, in the prior version you couldn't practice with Michael more, the game doesn't allow you that, you couldn't do it at the same time as rehersals and vocal lessons. So, you must do a lot of guitar lessons in this single update, no matter what.
On the other hand, it's true that in real life you need a lot of hours of practicing, that's why real life has a lot of boring moments, and that's why we play games, to get some fun. A better option would be allow you to do 4 hours of practicing in a row and not just one in a day. And if you want to go realistic, I think that the game is the more affected with that, a street hot dog in the game costs $40. And like that there are many other things that aren't realistic at all. And I'm fine with that, it's a game, it should not be a grinding experience.