> They talk a lot about working on the "modern web", that's marketing-speak for it uses established technologies that IE refused to use.
IE pioneered much of the modern web (AJAX...)! The problem with IE is that it got entranched in the enterprise and couldn't change very easily! Ya, sure, we get to go off and use Firefox and Chrome, but there are literally tons of internal web apps that only work in IE, making IE difficult to change and evolve.
> If I don't have to worry about my site working on Spartan, then Microsoft is in a position that it can start revolutionizing.
It is not actually about you, but about the corporates. Anyways, the same effect: now IE legacy has been tossed away with Spartan (the corporates can still use IE as a legacy app), MS can move forward without worrying so much about the past.
The approach taken previously, IE 10+ with a compat mode for legacy IE 6 web sites, didn't work well. Many just saw "IE" in the user agent string for IE 10+ and immediately thought "IE 6"! So Spartan lacks a compat mode, getting rid of that problem completely.
Honestly, it would have even been an improvement to take IE 11, throw away compat mode, and just call it something other than IE. I'm sure Spartan does much more than that since it would be a wasted opportunity otherwise.
(disclosure: MS employee, but far away from web browser development and speaking for self with only anecdotal knowledge about the history; also an avid Chrome user)