To show how the T-Rex isn't all that...I will take the commonly maligned "weakest" melee class.
Ranger (Beastmaster) 8
Pet: Poisonous Snake
Race: Variant Human 16 Strength, Feat Magic Initiate (find familiar for advantage once/round, and booming blade cantrip)
Level 4: GWM feat
Level 8:+2 str
Weapon: Maul
Spell: Hunter's Mark pre-cast and moved for first round of combat to target
Spell: Booming Blade, after first round of combat, where applicable
Poisonous Snake: attack, plus proficiency bonus plus poison
Pretty sure that build does around as much damage per round as the T-Rex, with a LOT more flexibility.
Treatmonk does the math here, but he does it right with adding in attack odds, crit odds, save odds, etc.. My back-of-the-envelope super quick calculation put it at above 50 damage a round with your type of calculation. But, you know, with a decent armor class. And equipment. And fully functional arms. And full player agency. And being able to think and not simply hulk-smash, no ranged attack, with a 2 intelligence which is probably so low you cannot even tell friend from foe sometimes.
And that's the weakest melee class. The new Ranger subclasses do better. Fighters, Barbarians, Paladins all do better. The spellcasting classes all do better too.
Polymorph is a great spell, just not the way you two are arguing it should be used.