To become a Grade 3, you need at least 40,000xp, at least 10 hours of flight time, and at least 50 takeoffs and landings. Once you achieve all of that, you have to maintain a clean record, making sure you avoid a certain number of violations within a certain time period. If you exceed the violation limit, you will be downgraded to Grade 2 again. To remain a Grade 3 rating, you cannot exceed more than 2 violations in one day, and no more than 3 violations in 7 days. These guidelines were put into place to ensure that nobody would get away with an excessive amount of trolling in the Expert server (which I think is your main concern?).
Also take into consideration that most people don’t fly on IF for the rank, they fly because they want to experience different planes and fly in different parts of the world with other players.
So while you are sort of correct that it doesn’t take tons and tons of work to become a Grade 3, there are still guidelines and limitations regarding the amount of rule breaking they can do. Keep in mind, if a Grade 3 decides he wants to troll on the Expert server, ATC will likely ghost him immediately.
I think that the way it is, the Grading system between 1 and 3 is pretty fair. Let me know your thoughts on that :)
With the release of Global, though, I think that the guidelines for maintaining Grades 4 and 5 are a little high… to maintain my current Grade 4, I need to conduct more than one flight every day. Grade 5s need to complete more than 2 flights every day… So there definitely is something that needs to be said about that.