No, the whole point of using a blockchain is to have a publicly verifiable ledger of all transactions. If anything goes wrong, there will be so many alarm bells ringing across the world that probably it'll be a hearable event for most people living in places that resembles modern civilization.

You can sell your cryptocurrency for real dollars before openly performing your 51% attack.

The idea that you could buy a real-world item worth half as much as the amount of ETH/BTC you would mine in an hour or two or 51% is simply unrealistic.

ETH mining at 500TH (a solid chunk over 51% just for ease) would net you 300 ETH an hour...that's 1.1 million USD, which means you would have to be able to purchase millions of dollars worth of real world items that would be released to you within the time-frame of your attack (in this case an hour) to make this even approaching "worth it" from a risk/reward viewpoint and even that calculation is ignoring just how profitable they'd be just mining normally for the foreseeable future.

No one is waiting for just an hour worth of confirmations for millions of dollars of real world goods exchanged for ETH.

Wouldnt 51% attack allow you to rewrite contracts?

no, contracts are immutable, I'm unaware of any base-layer chain attacks that can affect contracts negatively (outside of contract level logic that may be vulnerable itself)