By that logic why have Air Services Agreements at all? Operate like Africa / South America where 5th freedom / cabotage (8th) is the norm, no restrictions, the more competition the better.
The capacity restrictions in these agreements have nothing to do with airport slot availability, they are not there to preserve slots at the major airports. They are there to balance competition, and the lack of restrictions in regional airports (in many agreements, but not all) are to encourage new services to these airports which can do with the extra freight & tourism.
As above, nothing stopping them right now for updating individual agreements to give extra capacity to WSI. No need to make it carte blanche for all airlines.