The only way politicians will spend responsibly is if their available resources are actually constrained. Trying to convince them to constrain themselves with debt limits or budget balancing requirements won't work, so the only option is to use direct economic constraints. If cryptocurrencies develop such that taxation becomes unenforceable, politicians will be constrained by people's consent (one way or another).

I agree it will be volatile and messy, but a volatile mess of a vibrant and competitive currency market is far superior to the suffering caused when the state sponsored currency monopoly turns hyper-inflationary. I think it's unreasonable to assume that what's happening in Venezuela now can't happen anywhere eventually. I think we still have a good bit of time to avoid that in most of the world, but that's where we're headed if politicians are unrestrained and keep taking control over more resources.

I'm not sure how taxing marijuana would be considered more legitimate than taxing anything else. I've also heard auctioning off government claimed land as a fundraising option, but both of those would just be delays. As long as politicians can buy votes today by pushing costs into the future, there's not much chance of anyone coming into power who would spend responsibly. The only way to make a sustainable system is to make interaction dependent on consent. Externalizing costs onto others without consent leads reliably to resentment, conflict, and dysfunction.