By allowing you to double spend, it reduces confidence in coins you buy to zero, which reduces the value of all coins to zero. You still have your coins, but they aren't worth anything.
PS: Remember the value of Bitcoin is limited as a function of the cost of that 51% attack. If the price increases by 10x the transaction fees need to also increase by 10x or Bitcoin becomes less secure.
1. They have BEP printing press; instead of stealing or brute forcing into bitcoin and then selling the loot for $, the might as well ask Bernanke to print few thousands more sheets of 100 dollar bills (of course not legally but i'm sure there is some overprint like in any business).
2. screwing people out of bitcoins would mean screwing US citizens as well. Why would any part of government do that just to upset Congress and get themselves in front of bunch of congresspeople for grilling? Doesn't make sense.
Someone shorting Bitcoin on the other hand has an economic incentive to destroy it. The cost benefit of doing so scales with the size of their short vs the current hashing power. But, a malware writer may have access to a 51% attack briefly without owning any equipment.
There was a recent paper shared here on an article about new type of currency or exchange system. Although I don't understand details, it explained that since 51% of coins are already mined by just a few pools, if these pools orchestrate together, then can break the chain. But unsure how true this is (cannot find the post anymore, sorry)