Restricted space => creates fee competition => dust will become non-economically viable to move around.
Restricted space => fee competition
More adoption+fee competition => expensive fees
Expensive fees => stalled adoption
That's where the chain breaks.
Expensive is a relative term compared to your competition.
How much does a bank charge for an international bank wire?
If BTC costs < Western Union costs, Bank costs, etc = adoption continues to grow because your competition is still much more expensive than you. The money transfer industry won't go to zero fees overnight to compete with BTC.