Oh my god, please give me a try and let me straighten this out.
OP, I'll just state here first that WP8 devices will most definitely get WP9.
Backup for my claim: There are 2 types of setbacks for an update. Hardware and software. Hardware on the WP8 devices will surely provide enough computing power. Also, there aren't any software setbacks either. Our phones now run the kernel equivalent to the kernel in our computers and tablet computing devices (if on Windows, of course). This kernel is flexible and supports lots of computing power. This kernel is the same kernel used in Windows Servers. So, basically no setbacks in either hardware or software.
Microsoft did have a poor update history between mobile OS upgrades, including the ones from WM6.5 -> WP7.0 -> WP8.0. But there were setbacks that couldn't allow the updates. Unlike WP8, at least from what we know so far.
Back-up for claim: WM6.5 couldn't upgrade because most devices were very low-spec devices. A lot of the WM devices didn't quite cut the WP minimum requirements. Also, the screen resolutions were way too diverse. At the time of WP, I believe there was only one screen resolution which was 480x800. WM devices came in all shapes and sizes, including even square-monitor devices. Also, resistive touchscreens are an issue.
WP7 could not upgrade to WP8 because of the new kernel change, and how Microsoft wanted to avoid having the customers flash their own devices to upgrade to WP8 if an update was offered. If Microsoft did port over WP8 to the WP7 devices, all the users would have to flash their phones to wipe everything off and install the new secure bootloader. That probably wouldn't have been a great idea as the amount of devices that would be bricked from attempting that process would be high.
Finally, the modern UI known as "metro UI" will never be the reason for limiting an update to a phone, as long as most people in the WP Team at Microsoft have IQs over 50. If there was a design overhaul, the OS's UI would be overhauled. Simple as that. No, you won't need a new kernel for that. And besides, the reason why Windows Phone gained interest in a decent percentage of the human population was because of the new look. It doesn't matter if Microsoft changes to static grid-and-icons and removes typography, because if they do, most pros about Windows Phone on their own website will be invalid. Lumias won't look special or "beautiful" without the great combination of the software and hardware. So, basically scrapping the modern UI will both trigger suicide on Windows Phone & Nokia, and also will undo basically all the Windows Team's work on Windows 8. Hah, talk about historical rollback.