what are the downsides of voluntary taxation (https://en.wikipedia.org/wiki/Voluntary_taxation)? what would happen to a polity that used it as the sole form of capital allocation?

Too much complexity. As much as people bitch about the current process, one in which everything has to be self calculated is too much of a mental burden.

Chasing Curiosity. Empathetic Accelerationist. Founding Fullstack Engineer @icebreaker. YC S16, KB7. 
icbk.me/tayyab.fc

wouldn’t that mean that non-participants are simply non-beneficiaries? eg, don’t want to self calculate? no problem: you won’t receive whatever benefit you otherwise would have had you participated

yes, exactly “buying government” is the central idea

I’m willing to pay but idk how much to pay. What is the optimal tax ratio? 

If proportional, do people who pay more taxes get more benefits? That’s just buying govt.