Keeping old-IE compatibility prevents some optimizations and makes the code have to do extra checks, but using one file that supports it is simpler than using two files?
Ok, so the problems it causes are pain points for the jQuery devs and not for website devs? Also, the underlying code is more complex and lacks optimizations? Until old drops off the radar completely we'll have to use 1.9 solely or do a conditional check and serve the suitable version - serving 1.9 seems simpler ^^, call me lazy, many have,
Depends what you're doing. If you're building a mobile app (particularly one meant to be distributed via PhoneGap), or a Chrome extension, maybe you don't care about IE support.
Its also a pain for every website serving jQuery because of increased data for every single page load. Cutting out the support for old IE should also help with performance.
Not dumb at all. Keep using 1.x for everything until IE 6/7/8 drop off your radar.