Dumb? I guess if you don't factor in any sort of realism...I mean even now it takes months to years to acclimate folks to wearing sealed and pressurized rigs for space and deep sea operations, then add in usage in combat, more then triple the weight and I can see why you'd need to be dedicated heavy infantry to get the top shelf armor.
I mean from the snipets about alliance training we get, you seem to be pulled in differing directions depending upon skill set.
Today's tech for space suits is much more finicky and unstable, not to mention unsuited for combat.
Most versions of Sci-fi power armor are just a vehicle that you get into and drive, it does most of the balancing for you.
Kaiden finished his biotic training as a teenager, just the right time to join alliance training at the same age everyone starts anyway.
So what is the problem with training again?
The unholy trinity of archaic RPG systems like DnD is dumb and should not be taken as more than a vague balancing solution.
There is no reason not to make "tank" classes in different flavors, magic / biotic / tech included.