Tanks are supposed to counter burst damage. They are shields for the carry in that sense.
They are supposed to be countered by sustained damage like DPS. The problem I see lately is people playing tanks and acting like they can take all the damage in the world when the meta favors these champions that melt health bars, and even include true damage. They aren't unkillable meat shields anymore like they were a few months ago. It's why we are seeing less and less tanks and more and more fighters when it used to be the opposite.
What tanks and fighters have in common is they usually are good at engaging. It's easy to blur the lines between them, especially when the juggernauts come into play.
Tahm Kench is an interesting case because his oppressive power allows him to stay top tier in a meta that does not favor other tanks. Tanks in general are no longer the stars of the team anymore. The meta will shift again soon enough. I would say everyone should try Tahm in the though. It's a lot of fun.