If BTC is the only currency, it will go into deflation spiral, but since there are other fiat currencies, it will just get hoarded without causing too much problem
If some very popular product (like Iphone) only sell to BTC owners and reject payment in USD, that will cause a mass adoption of BTC
This sort of scenario isn't in bitcoins best interest right now... simply because the network won't support the volume needed for that level of transactions. After ASIC release - a the expected development steps to up the transactions per second we should be in the ballpark of being able to handle visa's transaction volume. Then will be the time to push for mainstream adoption by some super-entity...
But there's no reason it has to be a super-entity. I could very well (and imho more likely) be the miners that push/force mainstream adoption. For example: I'm in the process of building some cottages in Northern California (expecting construction to finish summer 2013) that will give large discounts for paying rent in btc. I'm shooting for 15% discount for bitcoin payments. I think I can do that in this location.
Maybe someone else will offer discounts for their fast-food franchises, chain of nail salons and other small business with multiple locations. That alone could push mainstream adoption quite quickly. Imagine the word of mouth advertising, your existing customers would tell everyone "you can get huge discount by paying in bitcoins" and those businesses would grow.
Then (of course) you'd have other business owners asking you "How can you offer such a deep discount?" and you'd get to explain it to them... then we'd see some real growth as it filters out into daily markets as a result.