I completely agree. There is a limit however.
Especially with all the new masteries, especially keystones, that Riot introduced, damage has been increased across all the boards - including Tanks' damage.
Then there's tank items which help them deal more damage, etc etc.
While the issue with laning is a fair point to bring out, another important issue to keep in mind then is a 1 item Sunfire Cape Poppy 4 hitting a squishy if she lands her combo.
I mean in the early game she probably does just as much damage as an carry, except she feels incredibly tanky throughout the whole early and mid game until the enemy starts being able to buy penetration items.
She gets to be tanky and deal damage. No tradeoff the way I see it.
I don't play Maokai often, but every single time I play him, by luck or whatever, I end up doing really well. And up until 3 items I'm just decimating the enemy squishys, literally.
How is that healthy when it takes like 15 AAs or 3 rotation of spells to kill me? Meanwhile I'm landing my combo on the enemy and see their health explode? (just a random example, i know)
Everytime I play a tank, and do well, I feel disgusted by myself. It's so forgiving to play a tanky champion, and your damage doesn't even fall off until well into the midgame/late game.
I do agree tanks should be even tankier though.