I don't think Metro, Or the modern interface, is what is holding WP back. It's nothing more than API/Dev access and OS level Feature parity. After that it's the basic chicken and egg problem; developers will want reason to make apps (larger user base) and users will want apps to adopt (more developers supporting the OS). None of which has anything to do with MS sticking to their design philosophy.
It's little things, like being able to choose any camera app as the default camera app, or developer API access to wifi points and so on. The predominant devices are all lumias touting cameras that are noticeably slower than other phones on the market. Toss in a lack of a value to feature flagship, missed deadlines and the constant feel of being a year behind in WP (people are celebrating wallpapers...). And then you have your reason for a possible decline of the OS. Not Metro/.design philosophy itself, but all that other gunk; speed, feature-set and accessibility.