This feels like a problem that could be solved by technology. It could be easily built into wallets. As long as the cost basis is known for the Bitcoin when it's added to the wallet, calculating the gain/loss at the time it's spent would be pretty straightforward.

I know this comes with other issues that aren't aligned with the Bitcoin ethos. But as long as we live in a world where the government forces us to pay our taxes in fiat then solutions like these will be needed during the transition.