People generally tend to overestimate the effect of install base. Especially for system sellers it doesn't matter that much, per the definition of system seller the ones wanting the game will get the system. And as both PS4 and Switch improved (and 3DS for that matter) in Monster Hunter week, it is clear that the game is a system seller. Also I think in Japan pretty much every possible fan on MonHun already more or less knows about the game series. So similar sales for World and Rise (which obviously is happening) isn't that surprising. We could have speculated if Rise as a game draws in more people, but it seems it is the same base.
Install base clearly made a big difference for Mario Kart 8.
I guess the difference with Mario Kart is that it's an extremely casual friendly game. I imagine this is why it scales well with the user base, whereas other core titles kind of have a roof based on how accessible they are and how much the entry appeals beyond the fanbase.