The can of worms of allowing other clubs to sue each other is massive.
Let's say for example there is a season that City won the league, where they are found guilty of enough charges that they would have been relegated....presumably the third lowest team that got relegated that season could sue them for 100 million. Up to 19 teams that season would demand a million (roughly) each for lost league position payments. 5th placed team that season can sue for loss of champions league money and team that missed out on Europa placement could also ask for loss from European competition.??
If found guilty over all those years they are charged, their legal claims would be into the billions.