wouldn't that just make it another Tank?
Not quite. At least, not back then.
Typically, a tank destroyer had what amounted to the next-size-larger cannon than its corresponding "tank" version. Because of the added weight of the bigger gun and its components, something had to give. In this, American tank destroyer doctrine was the opposite of Russian and German doctrine. The Russian and German tank destroyers eliminated their turrets and added frontal armor so that they could slug it out toe to toe with enemy tanks. The Americans dumped as much armor as possible on the hope that the resulting hot rod tank destroyer could run rings around enemy tanks. There were good arguments on both sides, but in practice it was probably safer to be in one of the German or Russian vehicles than in something like an M10 or M18, at least if you were in the Ardennes or a town.
I had a professor in college who was a lieutenant in a tank destroyer company. Towards the end of the war his unit was reequipped with M18s. He told me he was happy that he never was in one of those during a fire fight.
- oldman