Thats only a reason to cap the level to a total of 150.
Just look at the proposed 120 cap increase.
Now the problem is that this increase affects 1x100 and 10x100 characters. How would you balance this? Make it challanging for 1x100 characters, then the 10x100 (or like you propose 4x100) character walks through it and is bored again. Make it challanged for the 4x100 character and the 1x100 character is too weak.
And that will continue every time new content (or something like that) is added. The huge range which is allowed by multiclassing makes it impossible to find a point where TG can say "This is how powerfull we expect normal players to become" which is neccessary for making balanced and therefore enjoyable content for everyone.
Multiclassing also destroys any need of grouping. Why group when you can do all by yourself? And it destroys character individuality. With a cap of 150 or 200 levels, classes who multiclass would be different as they can't take all classes. But as it is now everyone has a fighter, a healer and a caster class. Those classes may vary (Wizard vs. Shaman etc.) but basically everyone is the same.
Sure,limiting the multiclassing now is impossible as it would kill the game. But when the game is succesfull again (or HZ 2 is made) multiclassing should be restricted.