I think about magic schools in the same way I think about science. If someone decided that Biochemistry would be merged into Organic Chemistry, or Cell Biology merged into Microbiology, I really wouldn't care because they fulfill similar, if not identical, roles. It's not really changing the way magic works, it just changes the way they are sorted. In fact, I would say that Mysticism breaks logic. If the spells within it are so different in function and use, how does one exactly become good at it? One can be good at Destruction because all of the spells do the same thing, destroy. One can be good at Conjuration because all of the spells summon things. In Mysticism, all of the spells do different things. Do you automatically become good at detecting life because you're good at teleporting, or moving objects remotely? No, I think there are better ways to group the spells in Mysticism and make them relate more closely to other schools. Hell, if they wanted to bring teleports back, you could probably fit them in Conjuration, because you summon yourself from one place to another. I think it's better to come up with ways for the Mysticism spells to fit into other schools than to let them sit in the island of misfit spells.
The way I figured it, The magic types were an inherit law in the universe of mundus just as gravity is also an inherit law. Each type of magic exploits it's own unique set of phenomena; restorative phenomena are unique to restoration, destructive arts are unique to destruction magic, etc. The reason you need to practice each class specifically is because each type of magic is unique, and all it's given spells are performed in the same way, with only the application varying between spells. I always assumed they were called "schools" because some branches of magicians specialized in, researched and taught specific forms of magic; just as there are specific research branches in our universitys, you can only educate new material if you discover new materials, which is why it would be a called a school of magic.
Integrating mysticism into other forms would not be very lore-friendly.
I don't really like Bullsh*t in my lore, it really bothered me in Oblivion that the explanation for Levitation spells being gone was that "the mages guild banned them" (how convenient) even though the real cause was because cities are in separate world cells. It just kills the game.