They fail to understand the purpose of e-mail, and as such we would never ever get the most basic and oldest of the e-mail client functions: folders.
That's a primary reason I stuck with Yahoo email for way too long: I didn't like the labeling system that Gmail provides as an alternative to folders. ("When all you have is a search engine hammer, everything looks like a search nail.") Finally, I decided to give in and use Gmail as my primary email service, labels and all. Why? Partly because Yahoo forced a new user interface on me that I didn't like, by shutting down the old version after initially allowing the old and new versions to coexist.
(Which makes Google's "Inbox" sound like deja vu all over again...)
I've tried Inbox a bit but haven't really given it a fair chance yet. My initial impression, though, is a bit negative: basically, it seems to be trying to solve a problem that I don't need solved. And with all the "improvements" it offers, it still doesn't even have folders...