I like the term "dumbed down"
negative or not, it's to the point. More complex features are removed or changed to appeal to a dumber crowd.
I wouldn't call it "streamlining" as that would imply higher efficiency. While it can be part of it, I don't think it quite covers all the bases like "dumbed down" does.
"simplified" could work, however it'd end up with a double meaning. Simple as in easy, and simple as in not complicated. Eventually "simplified" will carry the same negative connotations as "dumbed down"
Which then rules out just about everything. It will have negative connotations no matter what word we use.
Dumbed down is fine. Mechanics are dumbed down/simplified/streamlined so dumb/simple/ADD people can learn them. Interfaces are reworked or even removed entirely so dumb/simple/ADD people don't have to bother with them. The game is made less challenging so dumb/simple/ADD people can finish it, all while stroking their ego the entire time and giving them absolutely nothing to think about.
It's just what happens...the term fits. I say use it. Or whatever term you want. It all comes to the same effect in the end.