I would prefer more realism in aircraft production leading to slower growth and less over-use of spam-favorite aircraft (turboprops). Start the word in 1950 with hundreds of used DC3 type craft on the market, and then have a worldwide production queue instead of hundreds of airlines all taking deliveries every two weeks. Make it a small world with only 100 airlines with a hard cap of 1000 aircraft. Everyone can find a niche then.
You realize big airlines like American Airlines and Delta have 971 and 773 airplanes. Why limit?
Because the real world doesn't have 300 airlines that each fly that many planes.
One issue with aircraft limits is that it'll incentivize people to maximize seats per aircraft. So instead of 1000+ 737s/A320s, one who desires the capacity would resort to using A380s, 744Ds, etc. So limiting fleet size won't promote realism nearly as much as one may imagine.
Now if we were to also limit total fleet seat count, that would be a different story, but in principle I'm not a fan of imposing artificial limits to treat symptoms of a bad economic model.
I agree that a hard limit will encourage this, but if you have to wait in line with everone else to take deliveries of 747s, you will end up waiting until the end of the game to get all the planes you need to be that spammy. Meanwhile, the used craft will allow for niche markets (caribbean, europe, alaska, small airports) where you can be just as successful without spammy competition from the ultra-huge competitors.