Suppose iran now spends $10 billion and gets 100 jets. If the enemy this time turns out to be the USA, how much help will it actually be against this type of adversary?
First of all, you'll get 200 J-10CE with $10 billion dollars.
Secondly, who says you have to spend $10 billion dollars? Where does that restriction come from?
Thirdly, suppose that your enemy is a neighboring country like Republic of Azerbaijan or the United Arab Emirates. All of these countries are in fact more likely to attack Iran directly and launch a proxy war, particularly after what happened recently. Do you think you will win the war with just missiles?
Fourthly, even if your enemy is the US, 400-500 fighter jets with AEW&C and IADS will impose extremely challenging restrictions on the US that will discourage the US from getting into a war with Iran. It definitely beats the hell out of having no air force to protect your AD, just like what happened with Iran-Israel.
Wouldn't the US be able to just destroy most of Iran's fleet whilst grounded or even in the air considering the power of the USAF?
First and foremost, forget about the air, you need an air force to fight back
on the ground. If you are already so confident that you are defeated from day 1, there is no point in fighting back at all. You should capitulate and surrender unconditionally. Because without an air force, you cannot even transport your troops to fight the enemy, particularly in a large country like Iran.
An air force gives you the ability to protect your IADS, to prevent your enemy from establishing complete air dominance. In fact, no country can claim it has an IADS without having an air force.
Then, it obviously increases the cost of war with you (duh!). This is a no-brainer. That in itself is a form of deterrence.
I don't want to make a bold claim, but I will take the bait anyway: There is no such thing as true stealth technology if you have a dense, multi-angle, multi-static radar network integrated with airborne assets.
You cannot leave your C&C, acquisition and FCR, TELs, power supply, et cetera defenseless; and without an air force, they are defenseless. If you want to ask why, see what Israel did to our OTH and bi-static radars, S-300 FCRs, etc.
Of course I understand how useful an air force is against Israel, but please enlighten me if the opponent was the US?
No offense, but you asked this question already. How many times are you going to ask the same question?
I am genuinely curious. If the 100 planes would be useless against USA, then in this scenario one could argue that they would just be better off spending the $10 billion on more offensive and survivable weapons.
Completely irrelevant. These two things are completely irrelevant from one another. Refer to what I said earlier.
Here's the essence of your argument:
Imagine I am short-sighted and I can't see without glasses. And you say: OK, you can't see, but your opponent is strong and he can punch you and break your glasses. So, instead of buying glasses, you should buy a hearing aid instead.
And maybe just buy the amount of jets that you have space for under the mountains
Another weird argument. Why don't you build more underground air bases instead? Why is that not a possibility?