MD makes sense for a base set. It has the DP starters, it has cards like Call Energy, Phione, and Pachirisu. Not every set that becomes the base set is made to look like one, DX/HP for example, but to me MD does resemble a base set.
It wouldn't make any sense to me to rotate out anymore than GE. Like someone else said, and we've heard PUI staff members say this before in the past, they produce 4 sets a year. So it is only logical to rotate 4 sets out a year.
Of course this year could be different. I'm sure every year, there is someone who thinks there won't be a rotation. Japan had FRLG-on for such a long time, while we moved on to DX-on and HP-on. I see it being the same here. Japan keeps DP-on for another year, we move to MD-on, and then in 2 years, both countries switch to PL-on.