[ChadNauseam]

To execute that attack, you would need validators to sign blocks on your forked chain. Presumably they’ve already signed blocks on the main chain, so they’d be double-signing, which instantly costs them a very large amount of money on both chains (when you double sign, people can submit proofs of that on either chain, causing you to get your stake slashed.) That’s what makes the attack you’re imagining prohibitively expensive.

If you’re still worried about that happening to some ETH you received, wait for the blocks to finalize. Once they have, they will never be reverted without a change to the protocol.