All classes should have access to toughness enhancements, just not the same ones "primary" melee get.
Fighters/Paladins/Barbarians
5
10
15
20
Rogues/Bards/Rangers
4
8
12
16
Clerics/Sorcerers/Wizards
3
6
9
12
Something like this would be reasonable. They would still have lower HP than "Primary" melee, but their survivability will go up.
IMO at least.