Taxes existed with gold, fiat, and fractional reserve fiat. They will continue to exist with crypto. If citizens hide their wealth governments will just charge taxes on money spent.
But even banks have a lot to gain by adopting cryptocurrency and making money from the fiat<->crypto conversions. We're way too early to see either of these things though.
Adopting crypto would force them to innovate. Never underestimate the ability of lazy people to fight for the status quo. These are clock punchers that would have to become creatives.