I think the OP's point is that a lot of "legacy" users on Yahoo -- or on Hotmail[1], or on the free email they got from their ISP when they signed up fifteen years ago -- aren't inclined to move to Gmail unless they're tech-savvy enough to understand the case as to why they should. On the surface, free webmail services seem to be pretty interchangeable.
[1]: While Hotmail doesn't technically exist anymore -- it redirects to Outlook.com now -- I know more than one person who got their email account there back when it was Hotmail, still use it, and still refer to it that way. And at least one of those people is someone younger than I am and who's tech-savvy enough to be putting together his own PCs. Anecdata, of course, but I think a good illustration of why services like Yahoo Mail are likely to exist way, way longer than we might think.
Yeah, tech in general and especially email is annoying to switch from. My parents still use a yahoo email account because they have had it for so long and don't want to get other people to update their contacts.
The more traditional email interface on Hotmail/outlook.com isn't bad either. Gmail doesn't even show me the subject line by default when I'm replying.
So are you saying everyone should just get with the program and use Gmail? If so, how would this monoculture make the world a better place? To me, having multiple competing providers to choose from sounds preferable.
There's just no need, or space, for Yahoo anymore.