I cloud turn that argument around and say that there is an equally thin line between too little air defence and overpowered air units.
Overpowered air units are much less of a problem, because:
1. They could be countered with other air units
2. You still have a lot of other units, because air ones appear later
3. You still beed other units for things like conquering settlements
An interesting example is CivBE, where air units are clearly overpowered, but it's not an issue.
But I think that between the extremes of useless air units and overpowered air units there is a broad range in which a reasonable balance can be found.
There's another thing, the feature needs to be pushed to be very strong first, before such balancing. For example, recent patch forced us to finally use towns to max by making them very strong. They will be tuned down later.
Air units are far from this position. Most civ players don't play MP and in SP you could totally not build any air units at all. I usually don't build them on Immortal.
So, I really think that Firaxis need to enforce AI usage of air units first and only after players will learn to use them a lot too, could be the time for precise balance.
I agree that air units are expensive and a considerable investment. But I think they justify this investment by the power they have.
If it's already a question (it is, im many situations), when it's surely not an issue, which desperately needs balancing.
But this does not mean that this power should be without any risk. To the contrary, I think precisely because they are so expensive and powerful, their use should involve a risk.
In finances, all risks could be directly converted to costs. Having either risks or costs is enough, both aren't absolutely necessary.
I am fine with the fact that a bomber squad can easily take out a capital ship from afar. But right now, unless the enemy has an aircraft carrier of his own with fighters, this type of attack is completely without risk. And I think this shouldn’t be the case.
But what is the problem with countering air units with other air units? If you're way behind in tech you usually don't want to attack and for defensive it's ok to get air units later, less numerous and without commanders other than aerodromes.
I don't play MP and don't know how bad it is there.