Segwit2x had the miners support, yet it still failed.

It sounded like they required support from the developer community but they didn't have it. Not sure if that support would be needed for a 51% attack, would it?

Depends on the attack you're pulling off. A 51% attack won't allow you to change the coin generation rate (what peteretep was talking about).

Uncle Sam wakes up tomorrow and says:

"Compliance measures on crypto-currency are expensive. We'll be charging a 3% compliance tax on all crypto-currency holdings, and we'll do this via any US-registered crypto-brokers charging a withholding tax unless you can prove it's double-taxation; also any failure to pay this is tax evasion and will be prosecuted as such"

you'll see some hard forks. Also this is a _great_ idea as it increases tax revenue, increases investment in US bonds and stocks, _and_ it suddenly swells the value of US Govt gold reserves.