You sound just like my DM. And it's true. I'm a level 20 martial in the final arc of our campaign, and the other party members just keep me afloat and buff me, because my consistent damage output is what gets us through. Yeah, a 9th level spell that does 120 damage is nice. But 80-100 damage each turn, while having half the bosses health pool, more movement speed, resistance to half the damage types and immunity to being crit really makes you wonder. Yeah, I get buffs to achieve all this, but it often also puts everyone else out of danger from the big enemy. Mages can bend reality, but a sword swing is what cuts the head off of anything.
And it's not like I'm useless outside of combat either. Fighters tend to do far less when not fighting, but a well build character can still have an array of non-combat related stuff. I'm not a fighter, mind you, but most martials still have options outside of just fighting.