Hm, that's sounds highly probable and lots of people already don't pay taxes. You make a good point. With voluntary tax you could have a weak military. So the alternative is enforced mandatory tax. But could that be a slippery slope towards fiat currency and tax via inflation? I think you've thought about it this more than me, so I'm curious and open to listen.
Discussion
What about a hard fork in bitcoin that sets say 1% of all mined bitcoin and puts it in a pot, to be allocated in a way that is consensus approved in the hard fork (at a minimum; enforcement of transactions and remediation/ disincentives for bad actors). Not foolproof but satoshi wanted to go from 15-30% government spending to 0% which is kind of so beyond what we have evolved to cope with and also means there is no centralised force to combat other centralised forces (eg current governments) other than what people give voluntarily.
A hard fork is usually a very huge step. So we'll need to really think very carefully about something like this. Why do we need a hard fork in this case? How would this benefit people in differing regions with different governments? As for what Satoshi would have wanted, I haven't looked for any quotes regarding this particular issue. But does it necessarily matter what Satoshi would have wanted? He made the way, gave it away, and then went away.