“51% attack” is meaningless if you can’t find a way to profit on it. It costs money to run an orphan chain that long. You can’t simply double-spend because you are the biggest miner so long as everyone is running software that prevents it. To profit you must find a way to earn money with transactions on a given chain that are then wiped out when you release the longer chain.

The typical way to profit from this is to deposit to a centralized exchange, trade it for something else, and withdraw before the exchange recognizes what’s happening. However there are simple calculations you can make with the cost of 51% hashing and then the exchange can bump up the minimum confirmations required for deposits and limit the amount you can deposit for weaker currencies.

In general the 51% problem is no longer easy to profit from except with low quality exchanges.