This all assumes that you get paid in fiat and convert some to bitcoin.
If you can get paid 100% in bitcoin and live 100% off of bitcoin, why would you choose to ever use fiat as per Gresham's law?
Even a poor person being paid in bitcoin and spending only bitcoin will be better off for it.
Once you hit that, you can only "hoard" it up to a point. You still have to spend money to survive.
If everyone used 100% Bitcoin then the only deflationary pressure would be population growth which is hardly hyperdeflationary.
I doubt that Bitcoin will become the One World Currency so that people won't have any other methods of payment. Most likely BTC will be used mainly as a storage of value and people would still use their respective national currency (even if gone digital or blockchained) or some corporate backed system like Japan's upcoming "J-Coin", China's Alipay, East Africa's M-Pesa, Paypal/Samsung/Apple Pay, plus the traditional Visa/Master/JCB/UnionPay credit card or bank issued debit cards.
It also highly unlikely that people would be paid in bitcoin as tax payable are due in respective national currency. Not to mention the problem of how to record the salary compensation or for employees to report their income tax, given the possible exchange rate risk and potential capital gain.
The point being that if you want to design a crytocurrency which people would want to spend, it actually has to have some kind of "demurrage" feature like Freicoin. But something that evaporates your saving and force consumption would never see adoption, while a highly deflationary one like BTC has huge incentive for people to get in early and hoard/hodl it. So in the end, instead of something that liberates people from financial system, BTC behaves more like a digital commodity that fuels speculation and boom-bust cycle.
Now it is quite possible that if we were to succeed in building a new financial order, i.e. with Bitcoin at the bottom of "Exter's Pyramid", then we can start issue paper IOU backed by BTC and create credit and then derivatives on higher layer (which I think Wall Street is going to try in the next few years). Then people would have incentive to spend those BTC backed credit as a medium of exchange. It doesn't even have to be like the existing system, it could be Ripple type payment hub or a side-chain/new-cryto pegged to BTC and used only for payment, etc.
I have been paid 100% in bitcoin. Dealing with taxes is the same as being paid in fiat.
Funny you should mention Bitcoin as a reserve currency. While brainstorming a new floating island currency we were confronted by the hard truth that the currency would have too much volatility. One solution was to peg our currency's value to bitcoin. From our research we found that pegging the value to something like gold would cost a lot of money in holding a certain amount of gold, having it audited, ensuring that it was secure, etc. While using bitcoin as a reserve currency you can transparently show that it is backed, auditing is simplified and securing is easier than for gold. Bitcoin's price trend over time has shown less volatility, as it becomes less volatile than over a dozen national currencies. It is not a huge stretch to predict that bitcoin price could be one of the more stable currencies in the world and perhaps it could become a major reserve currency due to the ease of doing so.