I'm not sure that's true (see: Sparrow). I just don't think there is enough demand for people to want to pay for an email client. I imagine for a huge majority of people, default mail client's work fine enough for personal use.
Part of the problem is, unlike say Pinboard, an email client doesn't feel like 1-3 devs worth of work. The protocols are horrendous, you have to understand the quirks of lots of servers, etc. Plus all the UI and backend and search work. I'm not sure it's approachable as a small indie company.
There's also, of course, bad behavior by large companies such as mozilla: Thunderbird made it very difficult to build an indie email client because you have to compete on merits and against free, but they eventually got bored and just quit making it. Not to mention competing with free/ad-supported. And the semi-annual YC/vc supported email client company.
Not to mention what is coming close to active sabotage of productivity apps by the ios and mac app stores (lack of trials, lack of upgrades, etc).
"We now live in a world where people pitch tantrums about paying $10 for an app"
Where do users/customers get these expectations? Call it market forces, race to the bottom, whatever... do a little root cause and not blame joe public please.
It's still a prominent, maybe even the most prominent, item on my personal list of "reasons I shouldn't build an email app."
To make me switch, you would have to show me why the new thing was so much better. Like how Google search results were always better than Yahoo! (IMHO, no objective measurements used) so I switched. You would also have to explain it in a sentence I could immediately understand, whereas all the recent clients I've ready about either (a) claim to make things better but don't tell/show me how before I get bored of the page, or (b) use manager speak that I can't understand or is vague and abstract promises to increase collaboration or something. Therefore I've given almost no attention to any recent attempt to change email.
I use Thunderbird on Linux and Windows. It "works for me," in the sense that it's stable and predictable and doesn't lose my data. But it also has lots of niggly little problems that annoy me, like UI decisions that made more sense ten years ago than they do today and limited tools to support people like me who have a ton of mail.
So while I use and recommend Thunderbird, I also would love to see something come along that is so much better that it makes me fall in love with it, instead of just shrugging my shoulders and resigning myself to this being as good as things get.
Yeah, this is definitely what I saw with Sparrow. For it to become popular it has to be adopted by a substantial number of people. However, I find that only really tech savvy people cared about Sparrow and its streamlined interface. When I showed it to other people, they were reluctant to use anything else because they were afraid to move from their Apple Mail. I'm guessing if Sparrow had existed for windows it would've been the same thing with people using Outlook.
And then there's people who don't even use e-mail, who think it's outdated and prefer PM messaging as a form of communication. It's easy to see why a lot of these promising mail client apps don't really take off the way other apps and services do.
Nevertheless, Sparrow will always have a home in my Mac, as long as it keeps working on OS X.