I've dropped lots of games midway through, amidst suspicions they had a good ending, because they had a repetitive slog. WindWaker comes to mind, as it has a sailing quest that requires lots of driving across empty overworld in a straight line that you must complete to unlock the game's final dungeon. Under such a system WindWaker stands to lose money from me, since the time I would have spent in the final dungeon would add to the pay out.
Adding grinding to an otherwise good game could be a calculated risk, maybe. But likelihood of staying with it to the end goes down. Then my likelihood of recommending it by word of mouth goes down. My likelihood of replaying it goes down.
I think if the system was gamed, it would be from added postgame content, that comes after the games "a winnar is you" sort of moment and lets people who enjoyed the game an awful lot keep running the toll up. That might be a minor negative, some developer gets yanked away from fully debugging some section 40% of the way through the main story to work on postgame content that's not the absolute best allocation of resources. But I really doubt it specifically enables a strategy that makes more money from creating inferior games more so than status quo.
I guess to some extent what you're saying does happen with these trilogy movies that come out, and then the trilogy becomes a quatrilogy because they break a book up to make things longer. But each individual movie in those cases has pretty good quality and has to (the very last one, less so, like the postgame content) or else people drop the series altogether and criticize it. It's no blank check.