'mined' != 'confirmed'.  In fact a miner cannot really cheat in major ways if other nodes are doing their jobs properly and not cooperating.  Even if they were cooperating the team could create a god-awful mess and halt the whole system for periods of time, but they cannot steal or lose people's properly secured BTC.

Now if the group of entities who can 'confirm' transactions is small enough they could conspire to keep the system running but discriminate certain transactions.  e.g., transactions which were not deemed 'valid' by the authorities in the jurisdictions in which they operate.  In short, they could create fungibility problems within the currency system and I would go so far as to say that this would be inevitable.  And genuinely destructive to the value and usefulness of the solution.

The very people who were early advocates of extreme growth knew that the system would 'work fine' with only a small integer number of copies of the blockchain, which is necessary for real validation, worldwide.  They were also advocates of various kinds of 'color-listing'.  Extreme blockchain bloat was the path to get to a small enough number of 'confirmers' so that black/white/red-listing was doable.