I have a hard time to think of any reason how they could justify declaring it illegal.
Same goes for Bitcoin itself. But don't you worry, the US have already unleashed their Behemoth called FinCEN and their "experts" are probably thinking of ways to regulate it to death right now.
I saw the FinCEN stuff as generally positive. There were many other negative things they could have said and didn't, in particular I consider their delineation between decentralized systems and not as quite positive.
making bitcoin too big to fail would help
Exactly.
My general view is that Bitcoin is strongly supportive of the interests of the public and no equitable democratic nation should have reason to take adverse legal action against it. There may be some authoritarian dinosaurs who might try— but I hope and expect that they'll continue to be asleep at the wheel until long after Bitcoin is far too widely adopted by boring and normal people for them to successfully suppress it.
Trying to dodge policy with technology is something of a losing game here. It can be fun to think about, and there are many things you can do to try to keep Bitcoin more robust against powerful attacks... the most potent of which would be to constantly adapt. But mining is certainly a soft spot: the conspicuous usage of energy is a bit hard to hide, though not impossible.
But I don't think it's very relevant in any case: Sometimes people will compare a war on Bitcoin to a copyright-war or drug-war... but I don't think the comparison is apt. Illicit drugs still make the user high, unlawfully copies movies still make their audiences laugh— both of these things still work even if none of your friends are willing to partake. A money-like good is another matter— it gains value from people's willingness to accept it— and so even outlaws don't have much use for outlaw currency.
What protects Bitcoin is that its fundamentally a very just and wholesome technology, and not that different in terms of abuse-risk from cash which is still a norm even in highly authoritarian places. No government is a monolith, and some of the aspects of Bitcoin helpfully cut through a lot of gnarly public policy issues: It's really hard to resist inflating your currency when you are technically able— when resisting it means accepting some obvious negative outcome at the expense of some fuzzy long term principle. Sometimes to achieve freedom it's essential to take some options off the table.