I'm not sure that this is about the ESL, it's just more about the reality of the power that United/Arsenal/Liverpool have historically wielded at PL board level. The majority of supporters are unhappy because of the noise that Masters has allowed the aforementioned clubs to make about City. It has been a smear campaign, and it has worked because a lot of rival supporters think that the credibility of the league is undermined by City.
What Masters should've done after the CAS verdict, is the opposite of what they have done. He should've backed City, backed the idea that CAS found in our favor, and basically rallied around his product. Instead he chose to go to war on behalf of the red 3. Now he's paying the price as it has ultimately undermined his credibility and created serious unhappiness right across the division.
How do they prove what they need to prove? Even if City are totally cleared by the IC, rivals will continue to say City cheated. That is exactly what Masters should've been avoiding, not leaning into.