Why not just spend BTC and never pay gains?
I'm a U.S. person, residing in the U.S. I can answer the question on the basis of my personal outlook. The IRS has rules for barter transactions. I don't know what they are, and I don't want to know, but I don't want to violate any IRS regulation such that there is a feasible chance for the IRS to win a case against me in a tax court, at least a case significant enough such that they would not prefer to settle it out of court. (U.S. federal tax court is a kangaroo court set up by the IRS and operating according to IRS rules.) If I were inclined to activism, I might prefer prison to cooperation. If I had no dependents I would take higher risks. But it is what it is.
Prison is not so bad actually. It's a boogeyman. Everyone pretends that people who go to prison are highly likely to be anally raped,but it's mostly just a rumor used to instill fear. (Think about it -- if you had to live close to someone, turn your back on them, for 5-10 years, would you anally rape them? It would be suicidal.) If I thought the principle were sufficiently important and the ancillary consequences relatively less significant, I would violate any tax law I considered unjust, and brag about it. Those conditions have not been met.
Can I avoid prosecution for violating tax laws which, if the violations were exposed, would likely send me to prison? Almost certainly. But I think that if I am unwilling to go to prison to punish the state for its unjust abuses, then I have no moral standing to violate the law. Again this is a risk-preference. In this case it is moral risk which I am averse to. And again, were I an independent agent I might take more risk, but taking too much moral risk could affect my relationships in ways which I will not risk. As so many things in life do, at least for me, ultimately it comes down to sex.
Why would you pay the Gov for your BTC gains?
I certainly will not. I know the tax code well enough to avoid any taxes on BTC gains, in an entirely lawful manner, but maintaining the lawfulness component will prevent me from accessing those gains until I am relatively old. Meanwhile, I use them for philanthropic purposes (making them tax exempt, although I don't file the required paperwork because I'm pretty sure that's not going to create untenable risks) or for business enterprise expenses internal to my exempt holding structures. If I wanted to buy a lambo tomorrow, I would probably choose to pay taxes on the gains from selling coins. But I won't. I have different priorities.