Tanks are viable, in fact they are running a good portion of the show. Just look at tryko,dioraja,grypo, smilonemys, the 2 towers, etc. Heck Im sure indog2 could be considered a tank with how good he is at mitigating dmg (huge effective hp with evasiv+distraction stacked).
I think the OP is rather asking “what happened to tragod/stego/nodopato”. Well they just got replaced as people leveled up and got access to more “offensive” tanks. Whats the point of keeping a tank that mitigate dmg and hit like a wet noodle versus another tank that can mitigate basically the same dmg but can dish it right back?
Old tanks are still “viable”, but just much more limited in their matchups.