I think it is also fair to point out that he hasn't been just a drummer for a long time now. We can critique his singing, or his producing, or whatever, till the cows come home, but the reality is that he has been wearing many different hats for a lot of years now, and that is clearly far more important to him than improving his drumming.
Neal Morse joked in an interview last year I think that Portnoy's mind is a steel trap, that once he has played a song, he knows it and it's never getting out. He can not play a song for years and then just bust it out on a whim and still pretty much know it. And I think Portnoy even joked in the same interview that he he less notes to remember than, say, a guitar or keyboard player, but my guess is that translates to his on-the-spot playing as well when recording a new album. He can just sit down and play and get a take that is more than good enough to go on record (even if a few bits here and there need to be worked on a bit more later).