The deflation risk can be solved, I think, if the core developers and core decision-makers (key movers and shakers in the community, like miners and vendors) all agree to a "split" event moment where every 1 previous BTC transmutes into say 1000 subsequent BTC (or whatever, picking numbers out of thin air). Making it easier to get BTC because old values which were too close to the smallest fraction floor now become splittable and more easily transferrable again. The core folks driving Bitcoin at the moment are already well aware of this risk scenario and what the set of decent mitigation actions can be. This mitigation/evolution is not unlike when key folks in the Internet community realized that IPv4 would run out one day and thus something with a much bigger pool of addresses would be needed: IPv6. A similar hack is possible with Bitcoin too, or with any other e-currency or cryptocurrency. It's all just software. We can make it do anything we want, revise it, migrate to something else. Everything's based on trust and honor system anyway, even the notion of physical property ownership in RL: a home is only "yours" as long as your neighbors/community/government continues to honor that notion, or until/unless people with guns come and forcibly take it from you under threat of violence.

The lack-of-guaranteed-taxability risk is a larger one, imo. But I know it's possible to build a kind of wrapping/middleman service on top of Bitcoin, to give one example, if a government wanted to, and legally require citizens to use it in order to ensure their income/sales are tracked and automatically taxed (with fees taken out automatically to act as tax inflows to that gov). It's physically possible, anyway, without requiring any physical change to the protocol or blockchain. Just the Ages Old solution governments everywhere have used: the threat of violent force for non-compliance.