Probably a dumb question, but for what purpose do devs set up a predetermined coin limit (18m) ? Wouldn't that just cause people to hoard instead of use it as an actual currency?
This is one of the major FEATURES of bitcoin. Contrary to FAIT currencies, the limited nature of this virtual asset guarantees that its value cannot be inflated away by simply "printing more money".
For your second question I'd recommend reading about deflation and the deflationary spiral.