No one in their right mind is going to use BTC as a currency as long as they expect the value to keep rising.
You know your cash will be worth less this time next year, thats the incentive to spend it.
In the 19th century, during the golden standard, there was a constant deflation. So what? Why spend gold and silver, if you can buy more for them next year? Because you need to eat today!
Even now we have a deflation with a lot of things. Why buy a new smartphone now, if in 3 months it's gonna be much cheaper, and in 1 year it will fall to half its price?
If bitcoin users have all their cash in BTC they will have to spend it some time.