And I repeat, OP cost does NOTHING. It's not a hard no. And it's a BAD way to blance things strictly trough OP, because if I want a ship to be able to carry heavier fighters, I have to give it more OP. Which in turns means it will be able to mount more things on it and make it stronger in OTHER areas, ares I do not want, becaue OP doesn't JUST affect fighters.
It just creates more balancing problems, escalating in another direction, it doesn't fix anything.
At the end of the day, an option for a hard NO should be there, even if most vanilla ships wouldn't use it. Think of it beyond vanilla and look towards mods.
After all, if we follow your logic, there is no need for limiting weapon mounts - we can balance weapons trough OP cost too, right? Just put higher cost on a weapon you don't want the ships to carry...oh wait..
But if my example is not good enough, lets try something else.
Suppose a super bomber is 100 meters long. Should the freighter be able to carry it now too? 200 meters?
Say I want captial ships that can carry 4 frigates on it as fighters. Different types, so no built-ins.
Your ways of balancing means it doesn't matter how impossible or little sense it makes.
I don't see why one would be against this, since it's so simple to implement. Just a comparison between two numbers and a filter on mountable fighters based on it.