Id' argue Outlook UI changes with every version just as much as Gmail has. Gmail is also far less drastic, as updates are pushed regularly rather than a new version every 2 years or so. You can pay for GDrive and get 25GB for gmail.
E-Mail is a commodity and has been for quite some time. I don't see anyone disrupting the service side of it anytime soon.
They kept the old versions of their web interface alive for many years after the active development of the 'default' moved on to different designs. Which was very helpful for me, as I much preferred their non-Javascript laden interface and kept the one I started with for many years of use.
I only moved on when I tripped their anti-spam/account hacked filter and didn't have my business e-mail for three days because the messages the interface gave me were misleading. Apart from that, I was very happy with them and they were very sensible about what they'd do with the interface.
As an aside, I think $12 a year is unrealistic for paid webmail as you'd need a good level of scale to make it pay for such a small amount. You're already talking about a service a small number of people want (interface staying the same and willing to pay for it) I'd expect you'd find the same people would be willing to pay enough to make the service worth running.
You can hook up plenty of 3rd party native clients to it - if you want, you never have to use the web interface at all (9x% of my gmail usage is from Mac and iOS native clients).
http://jasoncrawford.org/2012/04/how-to-cope-with-the-gmail-...
The stuff I cared about anyway. I agree it is sad.
What I find interesting is that its even a product. I know that sounds weird but bear with me for a moment.
Mail (the service) has changed hardly at all for a decade. You send an email, may be you attach a few files, you get the email. The biggest change was that you could send text that had been formatted with HTML so that it looked 'better' (not everyone agrees with this) than plain, monospaced text.
Its a bitch to monetize in any way except direct payment, and yet the Eudora's and other mail clients of the world have faded from memory.
Given the collective memory, is it time for mail as a service? You know you pay $12/year or something and it just works. Nobody futzes with the UI, the amount of space you get/use is entirely a function of what you're willing to pay.
The question I'm asking is this, "Have we collectively come to understand the intrinsic value of email?" If so is there a utility service business plan that makes sense now? I could certainly see how you might structure such a business.