One of the main feature of Bitcoin is that the supply of 21 million BTC is know and cannot be changed. Indeed it is.

But imagine a world where bitcoin is mainstream. Banks will still exist! People will have extra money on hand, and will want to lend it in exchange of interest. Banks will be an intermediate to offer that service. I know a lot of Bitcoiners hate fractionnal reserve, but if it ever gets mainstream, not everybody will be against that. So even thoug the "real" bitcoin supply is fixed, the effective supply will grow.

Also, governments could, and will, hold significant bitcoin reserves they can release, of increase, depending of the market factors, to affect economy to their liking.

I seriously believe that the purported benifit of the 21million cap are overrated.