If I don't like a casino, I can create a bunch of TrustPilot accounts and leave 1 star and negative reviews about it with all of them, and that will affect their overall rating.
Are you serious about this? You think it's easy to make a review on TP? They are very strict and using multiple IP or devices will get your review deleted or not published. So I don't think what you are talking about will work.
There’s also the possibility of competitors buying fake reviews just to ruin the reputation of their fellow competitors just so they can get more people to use their own sites; that’s why I don’t trust the reviews on TP. Review sites that do their own research and personally testing regularly are more preferable for example CG and BTCGuso (From your response I don’t think you’re familiar with this site, but they are among the most trusted casino review websites here in Bitcointalk).