Tarkov studio claims it actually doesn't have the server capacity for everyone who bought the game for $150 to play its upcoming PvE mode, still wants players to pay extra

That is just wrong. For example, increased movement speed. Just check every couple of seconds if the movement that the player actually did is possible with the allowed parameters. Yes there can be bugs in that too, but not trivial to bypass when you validate every packages.

It is resource intensive. But much more effective for things like movement then client side.