I've not seen any direct evidence of a true deflationary economic situation (please fill me in if you know of one).
In the instance you claim you can buy 2 cars next months is very unlikely to occur, unless you bought into BTC early, and are hence being payed off for your risk, you are more likely looking at "buy one car now or two in ten years time". I know which I would prefer.
This may be true when Bitcoin's price stabilizes years from now, but it's certainly not the case now. And imho, it won't be the case for many more years. As long as BTC's market cap is growing, which if we want it to succeed we should be hoping for this, its price will be rising. And since the rate of BTC generation has nothing to do with the demand, the bigger BTC's market cap, the more BTC will be worth. That's the way it was designed. The faster the adoption rate, the faster the increase in value. Doubling in ten years' time comes out to about 7% annual increase in value. If this was actually how it worked, it would be great. But from what I've seen, the increase in value is much, much faster. And if you expect more people to use Bitcoins, the value MUST increase by a large amount simply due to demand.