> I don't think there is a need to restrict people to necessities when paying them a minimal income, they'll do that themselves.
I don't actually want to restrict people from deciding what to buy; I want to restrict the government and corporations from wielding the tool of economic incentivization—a tool that works in most of the market—near the margins, where it just becomes a punishment to no end.
Left to their own devices, UBI will be treated as "free votes" in the economy: it will be entirely soaked up by corporations raising prices, because people will still have all the same "non-free votes" they had before, along with the free ones, and everything will adjust to the expectation that people spend both.
Along with this, the government will continue to levy harsh fines, liens will continue to be applied, etc. In UBI, these will take away the money that was supposed to be people's social support. (In fact, in the "pure" UBI most advocate for, these will take away more their social support than is possible today, since we'll have also disassembled welfare, disability assistance, medicare, pensions, etc.)
---
Now, you're right in that people's needs vary. I don't agree that (pure) UBI is the best possible solution to this problem; it's just an easy one to conceptualize.
Do note that, of your hypothetical scenarios, I'd posit that the "fancy chair" is probably a medical device (and the entire healthcare system would be paid only in necessity-dollars; all demand there is inelastic.) And the book is probably a nicety. (Why not use a library? Unless it's a textbook—those are really, really inelastic, which is why they've gotten to be as expensive as they are.)
The flight situation is the truly confounding one. I would agree that this is something that people should be able to do. Yet most flights are for leisure or business, and it's nearly impossible to distinguish from any sort of government-verifiable context why someone is flying somewhere. There's no component of the economy-wide pricing of flights to pick out as being inelastic demand. It's a good example.
I'm not sure what to do about it, though. Even if you allocated people regular (nicety-dollar) UBI along with the necessity-dollars I've been talking about, that UBI would still suffer all the same problems plain UBI does. Flights (and anything else you could spend the UBI on) would just get more expensive by exactly how much everyone was getting.
---
Now, maybe I'm coming at the problem all wrong. The problem is that suppliers of goods with inelastic demand right now get to charge $(elastic demand + inelastic demand), whereas everyone would be better off if they could only charge $(elastic demand), and then get the $(inelastic demand) paid from some other source.
You probably don't need a separate currency to do that. You don't need UBI, either. You just need, basically, the cheapest house in a market to cost $0, and for all other houses in that market to be shifted down in price by that same adjustment. That could be accomplished with simple government rebates, probably. Find products+services with inelastic demand, write people rebates for the amount of that inelastic demand, done.