There are two cost verses capability gaps that need to be considered. The first is that between a disposable / not worth defending platform and a combatant able to defend itself, and the next is between a ship able to operate helicopters / UAV (and possibly STOVL) and a proper carrier.
The cost of the self defence systems of a combatant means that a corvette will cost you almost as much as a light frigate, even though it is the size of an OPV. Up gunning the OPV to corvette levels of capability is a waste of resources because it will not be survivable unless you add the defensive systems that would make it as expensive as a frigate. If you do not upgrade the OPVs defensive suite, the offensive systems you have fitted will do nothing more than make it a target and more likely to be attacked and sunk than if it was unarmed.
Once you have committed to a minimum offensive capability that entails a corresponding increase in defensive capability, it costs very little to increase the size to that of a full frigate that is more flexible, more capable, easier and cheaper to upgrade and maintain than a corvette.
When you then add the requirement for land attack, large ASW helos, AEGIS, area air defence, etc. you are repeating this process of making a larger more capable hull better value for money.
So if you want a minor combatant with a helicopter and antishiping missiles, that can defend its self, the sweet spot is a light or GP frigate, not a bombed up OPV.
If you want a ship capable area air defence, ABM, land attack etc. the sweet spot is a 8000 ton plus destroyer.
On the aviation side of things the magic number is six. The support facilities and personnel for a single aircraft is pretty much the same as for six aircraft. When hangaring helicopters on a ship, the largest number that can be effectively operated from a helideck and box hanger is six. Once you have a ship with six helicopters you have a target that needs to be defended. This is where a through deck design with its own defence capability becomes the more sensible and efficient option.
So long story short, the existence of the LHDs means the RAN needs more escorts than they have. Up gunning the OPVs is a non starter because they can't defend themselves and would need to be escorted.
