No because, when they did it Let's Play weren't as common of a thing on youtube. And The P4 one was special to the point that people still pester Jeff about it which has to be annoying. I don't blame him for saying no, just due to that. As much as I love Jeff and Vinny on the P4 endurance run, really that game was also just as essential happen the way it did. Few games ever come close to lending themselves as well to a MST3K treatment like P4 did. P5 is likely too well known to recapture the magic. Deadly Premonition was pretty good, and Chrono Trigger just didn't for a lot of reasons (lack of VA being a big one).
but Yes, because they basically still do them under different names for premium members.
Ultimately I just want to see them do stuff they enjoy doing, when they are having a blast is almost always when the best content gets created (e.g. when they played Cards Against Humanity on TNT).