Toughness is only a small part of AT balance. Contribution to team effort is a larger part.
Toons with Taunt abilities, especially AoE Taunt abilities, have something to contribute to the team besides damage. Toons with Support abilities or CC have something to contribute to the team besides damage.
Blasters contribute a lot of damage, a little CC, nothing else.
Stalkers contribute a lot of damage, a little CC, nothing else.
Dominators contribute a lot of damage, a good amount of CC, nothing else.
Scrappers contribute a lot of damage, a little Taunting, nothing else.
Sentinels contribute a little damage, a little Support, nothing else.
To me it seems clear that Sentinels need to be more correctly balanced alongside Scrappers and Dominators.