needs to grow up

I mean, the fact that Amazon owns IMDB is an obvious problem here given resources they've sunk into the film. Something like Rotten Tomatoes attempting to deal with Captain Marvel or Star Wars: TLJ review bombing is a lot cleaner because WB, not Disney, ultimately owns Rotten Tomatoes.

User review sites have become completely unreliable indicators for the casual fan

But streamers are a large part of the problem. Remember Netflix used to have a 5 star rating system. It never would have been truly objective, but you could have had a good faith attempt at a "verified rating" system.

The concerning thing is if Amazon's actions re: RoP is closer to Netflix killing a rating system giving bad reviews to prominent content they produced than it is a true anti-spam attempt.

The problem is the inherent conflict of interest between a review site and a marketing push in favor of a show if review site isn't buttressing marketing.

and they really need to have much higher standards for submissions to be accepted.

verified RT% sort of provides that. It massively skews ratings in a positive direction relative to a "objective polling" baseline (see cinemascore/posttrak) but nevertheless, the fact that it prevents review bombing does provide some solid value and ensures they're somewhat more meaningful especially when dealing with culture war adjacent films.