I don't understand what difference the WP kernel makes, especially since neither developers nor end users are directly exposed to that in any way. It's reasonable to assume that new hardware may require a newer kernel, but there's little or nothing to be gained on existing devices.
If WP8 brings along a different development platform, that might make a difference, but not in the short run: since WP8 is said to be able to run all the existing WP7 apps, I bet most developers will just stick to that, at least until the existing WP7 user base becomes irrelevant, and that might take a while to happen.
