If all that is required to reverse transactions is 51% control, cannot the transactions that occurred during the double spend attack also be reversed by a 51% coalition once the attacker loses its majority?
Sure but it doesn't solve anything. Remember that people do not trade token A for token A. You don't buy dollars with dollars. You use dollars to buy something else, like euros.
So you may reverse one token, but you won't be able to reverse the other.
i.e. suppose you have $100 and I have 100 tokens (e.g. bitcoin gold coins). You pay me $100 and I give you the coins. I now double-spend and sell the coins to someone else. You now have no coins and no money. I then double-spend that and give the coins to myself.
You could at some point fix this and get the coins back, technically. But you're not going to get your $100 back. Nor is the other person. And the other person never got his coins. So both of you are out of money, and only one has the token. Theft occurred.
Moreover, even if you somehow both had the coins, they ought to be worthless because the entire system is completely useless. If a system can be compromised like this, the tokens have no value. Just like a dollar bill has no value if it can be printed, or can magically be transferred to a thief at the click of a button.
I used dollars in this example, but the more likely avenue of attack is for the attacker to sell his bitcoin gold for other cryptocurrencies like bitcoin over and over. Like selling an expensive bicycle to a customer but keeping the key to the lock, stealing it at night and selling it to someone else, a dozen times in the span of a few hours.
All of this is a major issue without even getting into the political discussion on forming a coalition and deciding which transactions were fair, genuine, worthy to keep, and which weren't. That's virtually impossible, particularly when there's one set of double-triple-quadruple-spent coins out there to distribute with many people making equal claims that they were scammed.
