IE still offers the best debugging experience in Visual Studio. Trying to debug from Chrome or Edge is an awful, often bifurcated experience where you can't debug through from managed code to script, which makes tracking data flow problems incredibly difficult from client to browser.
Even though Edge is faster in many ways, it still falls down on simple rendering problems. Although those issues are now being resolved by punting to Google, it's disappointing to see that Microsoft is casting aside a browser that has already fixed many of these issues.
As a long time web developer (I still remember when IE4 was the GOOD browser), I completely disagree that adding <doctype HTML> is beyond most devs. Back in the day, this was the a problem because prior to HTML 4, nobody knew anything about doctypes, and if you did, you were probably using Transitional. In the intervening years, BECAUSE of the differences between IE and the other browsers, nearly all web developers learned that a <doctype> flag was required to know how your code was going to be handled.
Why not, instead of mothballing IE, just fully implement standards in it while in strict mode, and earn back your fans. Treat inconsistencies like event listeners, old AJAX calls, and css filters like polyfills and handle them - if encountered - as such? Then when a new developer opens up some old code that's running in IE, do what Chrome does and chastise them for using obsolete constructs? Shame them with "Does your mama know you still use filter:progid:DXImageTransform.Microsoft.Blur?" They'll love you for it.