I checked lolalytics by filtering for top/diamond, and there's a decent amount of tank picks in top lane with either good or respectable win rates.
Remember, solo queue naturally favors hard stomping and snowballing momentum. It isn't an actual representation of what the game is meant to be. Making it so tanks can have that power to hard win lanes and snowball like fighters and assassins get to will boost appeal, but will ALSO warp the game's meta too heavily in favor of tanks, which will stagnate champion diversity.
Even as it is now, tanks aren't doing excessively poor on average. However, tanks don't have an appealing play style in most scenarios and they don't fulfill an appealing role (peel and engage rather than hard carry), so they won't be picked as often even if they're in a decent or better spot in solo queue. Conqueror alone isn't "too much true damage" in the game, IE's true damage is an overall nerf to the item's early game, and there's not much true damage in the game that's floating around freely to complain about, anyway.
True damage isn't a concern. Burst in general is being addressed in a few months. I'd like Riot to finally be decisive about how they're going to address Enchanter game play patterns because very few people like or respect them, and all attempts prior have been half-hearted. That's my answer.