I think it's just as easy to make apps that will work on both 7.5/7.8 AND 8 as it is to make native WP8 apps so smart developers will do that, at least for a while, so they can have a much larger customer base. Also, don't forget that new low end phones in developing markets will be shipping with WP 7.8 for some time to come. Those people will want to buy apps too.
I would say perhaps a YEAR from now you will start to see new apps be mostly WP8 only. To go that route now, when you can make an app that runs on both and there are millions of people still using 7.5 and 7.8.
The first apps you will probably see that will be WP8 only will be apps that actually leverage the power of dual core processors and wouldn't run well on older phones anyway, like games with more advanced graphics, for example.
Eventually though, completely new apps will come out and at least some developers are going to want to make them work in Metro on Windows 8 and WP8, those apps are not going to work on 7.5 and 7.8, but I think we are at least a year away from there being a significant number of apps like that.