There is nothing wrong with CoTC, Poppy or other 'strong' tanks
Game times are even shorter now, with average games ending at 25 minutes.
Tanks have incredible mid games, what with they stack health and armor before the enemy has the items to deal with them.
ADCs have bad mid games, so they cant deal with tanks who have a
+
when they only have
+
or
+ 
if ADCs, assassins or mages picked up penetration items such as
earlier, such as as a second item, then they could deal with "the unkillable tanks"
TL-DR : Tanks are strong right now because shorter game times increase the amount of time they spend at their power spikes compared to their counters, and the classes that counter them refuse to change their itemization pattern to adapt to this. And I'm ready for the downvotes for the people mad at only reading the title
and your damage is literally lost. that