D&D 5E Stacking martial classes levels for extra attack feature?

[Scott Steven Bell]

I simply would not allow it. Someone taking ALL those classes is hard to fathom. How in the world would one guy have access to learning all those classes one right after the other? The player would have an incredible amount of abilities from all those 1st level booms.
Im a very open minded DM but you have to give the guy that plays a class up a few levels something to balance out ALL the abilities that the multi classer gets.
Sorry, never at my table