Nah. BT as tank will also last you until platinum.
On offense, no doubt BT is one of the best. Even better than Ares sometimes (I use both).
Kashrek is an annoyance to people. Yeah he is not great only heals himself and his flanks.
But I like to troll people when they actually lose to him. I do often put him at tank (win rate is between 20-25% most of the time)
BT still useful in diamond arena due to his attack buff but definately not as a tank.
As for Kashhrek, I’ve got him at 3 60 but I decided to max Caedmon instead. A decision which I never regret.
Yeah my defense team doesn’t last long in Diamond at all but my offense has no problem getting there with BT in the mix. And I dont like Kash too much either. He was stuck on 3.60 for a while but I wanted to strengthen my 30 hero roster for war.
Kashrek forever. He is the best 4* tank to have.
True enough you do need to flank him well.
Kash flanked well can keep you in high platinum or low diamond.
Kash exists in @Razor 's hero utility for a reason. 
Buddy is a way scarier 4* tank.
Boldtusk, Kiril and Rigard are all better than Kashrek IMO too.
I even find Colen and Hu Tao scarier than Kashrek because if you screw up or get a bad board you can lose if their special goes off once.
Rigard. Kunchen is tank material but if you dump emblems in Rigard you’d end up with the stats of a 5 star. I’ve seen one at level 11 and he took shots from both my maxed Joons and still lived.
Kash can’t do things by himself. A healer tanks job is to stall time. He must be flanked by good hitters. And he does his job really well for a 4*.
