I don't get it (Big...)
Can't IE mode just be baked into Spartan, with a "switch engine" button/intuition that remembers if a particular site needs IE or not? Why not make this offer ie6+ modes for full-on legacy support. And how about a similar concept to handle Java needs... and, er, cringe, flash?
Hmmm... wouldn't that make this actually "attractive" for business users? Bundle it all up in a mega secure sandbox with paranoia settings and you've got something for everyone. And release it on w7 as well, with a guarantee that anything that works in Spartan on w7 will work in w10.
Of course we want to abandon legacy crap, but sometimes that's easier said than done.
The "unique selling point" of windows should be legacy support, especially for business. When I upgrade yes I want a little shiny, but I want all my old stuff to keep working, otherwise it's not an upgrade, it's a "change". Change is hard. If businesses are to change to w10 before 2021 then ms need to shift their focus to where it matters before all our software is "cross platform"/cloud and we, shock/horror, don't need Microsoft any more...