I don't know if you've ever tried tanking ifrit, valk or bee using a 99 def tank before, but it's the other way around. It's virtually impossible to tank ifrit and valk with a 99 def pally but still possible to tank bee without too much trouble if you have the right gears. The only way you're gonna survive tanking either ifrit or valk is with constant sw from your party but that defeats the purpose of having a 99 def tank cos any other class would be able to survive better with constant sw while wearing equipment that's more suitable for the situation - even if you spammed slims non-stop as a 99 def tank it's not gonna save you from getting one-shot from ifrit's powered up flame breath for example.
But yeah, 99 def tanks are still a viable build, just much more limited than what they were able to do before. It's very difficult to predict whether or not they would be useful in a certain situation using the calculator or formulas we have on hand like % decay per non-boss/boss monster like Black Metal tried to do, because other factors come into play such as mob skills. Until someone comes up with a clear-cut and accurate way to make predictions, I'd say the only reliable method to confirm its viability now is for someone who already has the gear set to test it out.
Just as a side note even though it's possible Naght is really difficult to tank using a 99 def pally/lk without constant sw, that it's just not worth the trouble compared to a sb champ.