In regards to young players seeing game-time, there are a lot of great coaches who have the philosophy "When you show me you can do the things I need consistently in practice, that's when you get floor time". Obviously, none of us has been the practice regularly and had a chance to see exactly what BJ is demanding that Thompson and Thiam be able to do before getting game time. As Eco mentioned, throwing unprepared players into a game, doesn't really make that much difference in their development. There are a lot of coaches that feel the same way.
As I mentioned, haven't been to practice to actually see for ourselves, but what if TT is getting absolutely dominated in the paint by Curry and Daniels (who aren't even Honorable Mention caliber bigs)? What if the standard Johnson is looking for is "just be defensively competitive every day in practice and show that you're capable of stopping your teammates in the paint x-percentage of the time" and we'll try to work you in.
Again, not being at meetings and practices, but based on some of the things Johnson has said and his reactions to some questions from media, it seems likely that Ben has had many conversations with Thompson with the basic theme, "You're shooting for floor time next year. This year is about gaining size and practicing against other college players". If Thompson is truly serious about wanting to be a Gopher, pretty sure he knows and understands that he needs a lot of work in everything except maybe his 3-point shot to be ready to contribute. I'm guessing that Coach Johnson is being very careful with Thompsons minutes and trying very hard to only put him in situations in which he's fairly certain that Thompson can have a positive experience as opposed to putting him in situations in which (based what he's seen in practice every day) it's likely he'll get hammered. Good coaches also tend to keep that in mind as well, when doling out game time.
Now, if you are one of those who believe that the ONLY way to be ready to play in games is to actually play in games, then you'll probably disagree with this post.