The Bishop: No longer needed?

The whole debate rests on one crux, that simply being player availability. The only reason why Bishops, ArchMages, and several other classes still exist is because they're needed at certain times for certain things.

Wrong. Most people play their classes because they like the playing style. If they find out they don't, then they switch classes. Or they can follow their default and stay with the class they originally picked. It has nothing to do with what's available or not.

Okay, so we know that simply to keep playing this game or to do essentially ANYTHING you need the help of other people. Now, we know too that you can have 30 NLs in the Zakum Altar and have a run that lasts 3 minutes. What a great way to work, no? 3 minutes is all you need to play this game every day...what a deal! Oh wait, there are never 30 NLs all in one place at the same time unless it's organized, and even then, having a permanent time for all of them to meet up is bound to create problems for every one of them sooner or later (aka some will be unable to come from time to time). So who do you substitute them with? Heroes, BMs, other classes...oh wait, there aren't any of those available because it's a 12am PST time slot (you see where I'm going with this?)...so what's still available? Those other classes - Bishops, ArchMages, Shadowers, Paladins, etc.

While this is possible, not everyone is crazy about Zakum. Your friends aren't on, great. Go kill Pap instead. Oh, you papped twice? No problem, training will take it's place.

Also, don't forget that the reason people don't do 30 man runs is that it lags like hell and the split is near 0 (including the exp being split among 30 people). You don't substitute, you just stick with it and go without them. People nowadays do mostly one or two party runs, and its not hard to get ~10 people in and out of the altar.

Life doesn't work that way. So we have tons of other classes from which to choose from, and millions of people who play around the globe. Together that makes a humongous array of options 24/7, none of which are the most optimal or efficient, but all of which are capable of success.

I agree with this part of your post.