If someone wanted to cheat on the last block, he would have to keep on mining all/most future blocks.

As soon as they would stop or a good miner would take over, that chain would be the truth and undo all blocks that were falsely manipulated.

It is generally assumed the energy needed to falsify transactions with possible gains that can be made (double spending of owned #Bitcoin is one of the few things possible), and to maintain that chain will cost way more than can be achieved by doing the honest work.

Especially since if the false chain was broken all energy spend and all gains would be gone too !

Any wrong doing would be noticed and why would people keep using a monetary system that is known corrupt (plenty if I look at fiat 😇).

And a hard fork would be an other defend mechanism.

The only real reason to attempt such thing would be completely destroy Bitcoin.

Most argue even state actors won't be able to pull that off in current reality.