I think people underestimate the cost of switching users and how much people beyond a small scale will readily switch in the b2b space.

Despite some people opining about how email is broken, the fact is that people prefer its simplicity and universal access.

As a team lead I tried to start us on Asana which I use for personal projects and I think is a fantastic product. I assumed people would love it. To my surprise only a few actually signed up (reluctantly) and after two weeks the project was completely silent and people were emailing to ask questions or check for updates that I had already submitted to the Asana project. After asking them to check the project enough times I gave up and switched back to email.

You could look to replace either it's all about what you see as the problem. Does your spouse email you your shopping list? Do you email yourself a to do list or do you write it down in something like Remember The Milk?

Yes, I believe that you could but founders are still going to try for both.

Inbox-zero isn't so much zealotry as a method of managing to-do lists. In the business use case, there seems to be a tipping point where the alternative is that other people are responsible for things they send you, which you might drop because they get buried into your 10,000 other e-mails and by the time you get to them, you might miss some. If you're in a culture where task assignment can happen through e-mail you're almost forced into some variant of inbox zero -- particularly if people aren't going to be following up to check on your progress on small tasks until they expect them to be done at the next meeting.

Good question, would seem there are enough to warrant the creation of quite a few email products that are supposed to help you get to this "zen" like state.

One of Mailbox App's pitch points is even along these lines: "Inbox zero. Daily.".