Like any other business you have to pay for whatever you need to meet the expected work flow. For your example if a dental office can't afford the extra $327/yr to have that 16SC capacity then they are probably going to go out of business anyways. Any other system that charges per extension instead of per SC is going to cost more anyways. 3CX's pricing model is fair and is cheaper than just about any other commercial alternative.
Besides, you are anticipating a problem when there isn't one. Offices have been paying this price for years long before VoIP. Back when you had analog lines or a PRI, you had to pay for that capacity 365 days a year, and it was many more pennies then than it is now.