It's possible, but from my recent experience with tournaments like Rizin, TUF, Bellator, and the PFL so far, the quality of fights doesn't seem to diminish. In fact sometimes you have guys trying to end fights quicker to stay healthy, which can make things exciting. I'm not just saying that to counter your point, it's just that every recent tournament that comes to mind, like Horiguchi's 4 Rizin fights in 5 months, seemed to work out smoothly. It feels like promotions have figured out how to deal with pull outs, so even when there is a injury it doesn't seem so bad.
Once PFL presents its top 8 of each division (as well as their playoff brackets), it's really just 2 days the fighters need to show up for. Once in October, and once in December. It doesn't seem likely that all 6 tournaments will fall apart, and because of that, PFL is pretty much guaranteed at least a couple of properly set up million dollar fights for the final episode of the season.