Or government could shut down exchanges that allow conversion to fiat. Then let's see how much you can really buy with crypto.
Performing 51% attacks in the hope of discrediting an asset would be a waste of resources. Such an attack on the Bitcoin ecosystem would nowadays cost several billion in hardware and electricity, and all the community has to do in order to repair the damage would be a hark fork. 51% attacks happened in the past and most of them had little to no impact on major assets.
Literally irrelevant for any non-PoW coin out there aka the majority of all coins.
The majority in terms of number of altcoins that have a forum post? Or majority in terms of converted USD market cap?
Does it matter? Its basically just bitcoin that makes up almost all of PoW.