1) Tanks have never been good because they "did little damage". If you seriously think that tanks were good and did little damage then you simply didn't play in Season5/Season6 tank meta. Tanks did a lot of damage back then. Not to mention even in S3/S4 Mundo/Shyvana top meta people bitched about how tanks just 1-shot them without taking any damage.
I will agree, tanks were tankier, but they were still clapping squishies with ease in the early-mid game. And that's what made playing tanks then "so fun". I remember player Nautilus and just building full tank, and absolutely decimating the enemy ADC while I took 0 damage. Same with champions like Volibear, Maokai, (rework) Poppy etc.
2) We can see what happens when a tank trades their damage for CC in Zac. It feels absolutely awful to play. You can't kill shit after a certain point in the game, and you have almost no potential to carry or punish poor positioning unless your carries are in position or have the items to punish.
In _any_ coordinated environment where you can rely on your carries (such as Clash), not just LCS/pro play, Zac will be super good. However, in SoloQ? Zac will be trash and super unfun to play. Old Zac used to be super fun because you actually had carry potential. I could snowball and have the ability to 1v1 people in the enemy jungle with my lead. Zac did _way more damage_ and that's part of what made him so much fun. Now even as a 3/0 Zac it'll take me 20 seconds to kill the enemy jungler. It just isn't possible if my carries aren't with me.
3) Tanks doing 0 damage just isn't going to work (at least with top lane tanks). Tanks will never be viable in the top lane if they don't have any power to 1v1 in lane or get push priority at any point. "Just make em tankier lmao, they shouldn't be able to 1v1" will not work. A tank can never group in a 5v5 if they can never outshove/outtrade/out 1v1. So either you give them extra minion damage and make them have a super uninteractive lane phase where they just afk shove the wave, or the tank is forced to sit under their tower all game hoping to god that the enemy laner makes a mistake so he can group. Cause otherwise he'll lose too much XP/towers for any roam.
The way tanks used to work is that they actually usually won lane, and had to impact the map before they fell off and would lose pressure in the side lane. This Hashinshin-style "they should never win 1v1" doesn't work in League. Unless ofc you want tanks out of top lane permanently (like Hashinshin).
**4) Making tanks viable by buffing their damage actually lowers net damage.
People are acting like the game is 72 champs vs 72 champs. It's not. It is 5 champs vs 5 champs. A top lane Maokai even with a _30_ damage buff on Q will do less damage than a top lane Renekton/Darius/Riven/Kayle at most stages of the game and be more durable. The 30 damage buff could make him more viable and more fun to play which'll put him in the meta and actually _lower_ damage in your games.
If anything Riot did try and make tankiness/clear speed buffs to Sejuani/Ornn after gutting their damage, but quickly realized that in SoloQ they still wouldn't be popular until they did more damage. And the only way to lower net damage is by making more people pick tanks. So counter-intuitively the way to lower damage is by raising the damage on tanks.