Oh dear.... This rule has nothing to do with realism. It is not anymore expensive to keep 3 fleets if you add one. The cost goes down per plane / for bigger fleet, but if i have like ATR, B777, B737 and I add a330 it does not add xpenses for the three. This sucks so bad and is the worst problem in the game.
Disagree. There needs to be a balance mechanism in the game to keep people from "flying all the things" and that forces them to think for a moment about their fleets. If everyone could just fly as many fleet types as they want, the "big boys" would dominate the game even more and there would be no niches anymore for unexperienced players like you to be successful in. Let's assume you want to play in ORD, but there is a huge airline already. You would analzye their fleet and see that they fly A321s and A330s as an example. What does that tell you? They cover both domestic demand as well as international demand and even cargo. What you also see is that they operate quite big planes and probably neglect the regional demand. Even if not, that airline would be very vulnerable to the ankle biting technique where you compete with some regional planes vs their large A321s. If that huge airline had no commonality restrictions, they would probably fly regional jets themselves to avoid being attacked/being vulnerable.
Another example. You join in 1975 and see that the local airline at your airport flies a mix of Caravelles, Comets and DC6. What does that tell you? They have 3 fleet types which are all getting more and more obsolete in this era. This again means there will need to be transitions soon if the airline wants to stay relevant. If you can grow at the airport and compete with this airline, you may reduce their profits just enough so they cannot afford the transition (going 4 fleets while paying for new planes as well). Suddenly you bankrupted your competitor and have a monopoly.
Short: You don't know what you are asking for. The limit is in place:
1) To limit experienced players/big airlines from being overly dominant (even more so than they are now)
2) To give new/unexperienced players like you a chance to analyze the market and find a niche
3) To give the game some more strategical gameplay where you have to think about your actions and their consequences where the most efficient airline profits
Don't see it is a restriction, but as a chance.