Making Black Cleaver stack slower won't solve the issues that tanks have

making it stack slower would probably make tanks last a bit longer in team fights since it will take 3 seconds, if the numbers are the same as the ones in the dev diary, for tanks to lose a quarter of their armor now. But they will still get bullied out of lane by the fact that most champs are now rushing an item that takes away from their primary defensive stat, and since they lost so much lane pressure after the nerfs to sunfire it's incredibly difficult to lane properly once that item is built. At the same time, tanks will still be stuck fighting team fights with gimped armor values, even if it takes longer to stack. I have 2 suggestions that would probably work better than slowing the stacking of black cleaver, if you want to bring tanks(real tanks not ) back into the meta

1. change it to a shred of bonus armor but increase the value of it so late game bruisers won't be losing out on tank shred, but tanks can survive lane better, and can atleast have the option of buying a health item early, instead of losing whether they do or don't

2. Buff the early game of all tanks that are currently struggling like change the shred to a self buff granting armor pen working the same way as it currently does but instead of debuffing an enemy it gives you the pen, so it loses out on its team fight utility. This way tanks are still weak to bruisers in a 1v1 but don't need to worry about the adc getting to ignore their armor until they get a last whisper