I'll probably get downvoted for this but.
If you want to maintain "compatibility with older versions of IE" try an older version of HTML, an older version of CSS, an older version of javascript and an older x86 processor, and an older version of windows (95 sounds nice).
Sorry.
If future features should try to remain compatible with older versions of IE we will get very little innovation.
So the problem will not IE versions that are already old (IE6, IE7) but current versions of IE - even future versions before they implement these new standards.
As these features roll out to Firefox and Chrome, users of these browsers will have to them because of the auto-update pretty quickly.
Eventually IE will implement them, maybe in version 12, and you'll have users still using IE10. Heck, maybe they won't be able to upgrade their IE because it won't run on their aging Windows 7.
Unless IE switches to the auto-update used by Firefox and Chrome, and stops dropping Windows versions in their new IE, I don't see the problem going away.
Of course, its those creaky old IT departments.
Stop catering to these old browsers and people who want your app will download a capable browser or pressure their IT or just use their phone.
If they can deal with Y2K, they can deal with something this trivial.