1)Samsung is not going to stop using android especially when it is now becoming very influential on Google. Samsung consists of over 40% of android sales. this puts them in a position to force leverage on google and increase their revenue. Changing to tizen and letting that opportunity go would be like eating the Golden Goose, they will ride this wave until they either drain google or a better OS comes along.
2) There will be AOSP roms for GS4. A developer will come along and see the gap that Cyanogen mod has left and will fill it , if not for the fame then for the donations, either way a top tier device is not going to go without development. Its silly to think otherwise.
3)I don't understand why someone would want an AOSP rom on a Galaxy Note 2 or a GS4 device, totally criples the device. For all the Smack talked about touchwiz there are some features that i wouldnt want to do without. I had a Gnex and i wouldnt go back from my Note 2. And this is not my Fanboism talking since Gnex was a Samsung product.
My note 2 runs better with aosp so i don't agree. As far as cm10 or aokp for the s4 its a wait and see deal. I wouldn't buy the phone just because of a rom. Your right we will get roms but im thinking most of them will be stock based.