We had this problem too at Inkling (W06, https://inklingmarkets.com). Classic problem to bump into. One thing to alleviate this was to just introduce a clean slate leaderboard at some time interval. Like a weekly leaderboard. Which seemed to help a lot bring back the competitiveness even though there were these all time leaders of course that were with us from the very beginning accumulating the most points. It's all what you choose to show folks.

And yes, it's hard to pull away things from those vocal early users of games. Like maybe ditch those overall leadboards you thought were so great at first. But it's obviously in the games long term best interest to not be completely beholden to the loud, early users if you made a mistake. I remember we also added game mechanics we had to cancel in other games we developed (Cityposh, S11) because they just gave a few people too much manipulative power. It sucked to make a few angry about it, but it was better for the majority of folks playing.