Don't people who use their BTC to buy from Bitcoin-accepting merchants usually replenish their lost BTC after a while? That would create a net neutral effect on Bitcoin prices.
They can't do so forever. Depends on the respective individuals. We'd have to figure out what the typical Bitcoin customer is doing!
Sure they can. Many early adopters intend to use bitcoin as a currency,
and to hold it as a long term investment. Doing both at the same time means replenishing what you use as you go along. When I buy something online, I use bitcoin if I have the chance, but I prefer to deplete my USD savings rather than my bitcoin savings, so I do a quick USD -> btc -> purchase. If it's a miniscule amount then I might not bother with the USD -> bitcoin step, but if a bunch of miniscule purchases added up, then I would still eventually replete my bitcoin holdings.