Maybe after WP8 gets some sales, developers will start writing WP8 only apps. The developers that created the currently available apps will most likely continue with Silverlight and XNA because it could be a royal pain to start migrating your current code base to a completely different language. And since Silverlight and XNA are still supported, and most probably will be that way for all time, there is no real reason to start migrating.
So maybe the WP7.8 phones won’t get those fast C/C++ apps, but the current crop is pretty satisfying and it will most likely still grow. Most major apps are available and with the customer satisfaction rate of Windows Phone, it’s hard to think that people will care about not getting the WP8 update.
But new purchasers will most definitely care about it. Most people will think “why should I buy a WP7.5 phone now and not wait for WP8 phones to launch?”.
Well the answer is simple: by the time the new apps designed specially for WP8 will have a significant impact in the Marketplace your phone be it WP7.8 or WP8 would have been replaced by a new one. It took the Marketplace two years to get where it is today. Keeping in mind that current devs will not be switching to the new language anytime soon, another two years may pass until the number of WP8-only apps will become relevant and a purchase decision factor. By that time, your current WP device will be surely replaced by a new one, probably a multi core device.
Anyway, this is my opinion. If you have any comments on the matter you can leave a comment below.