Like some have said, the complexity of modern apps is more to do with the target for them at this point, which is still touch first but once we can have Windowsed modern apps this thinking changes. Since the store already shows you desktop apps, it's not an issue of a developer to list that their app needs a mouse+kb to work. You'll see more "beefy" apps in time, I have little doubt.
George P - I don't buy that. I think the REAL reason that ModernUI apps are largely so simple is that they were designed largely by mobile-app developers - who are used to thinking in terms of uncomplicated apps. MetroIRC and MetroTwit, for example, are two of the more complex ModernUI apps - and both were designed as UX-neutral from the beginning; further, neither excludes any method of interaction - unlike HootSuite for example, MetroTwit didn't require a mouse at all, or touch for that matter. (Yes - you could use MetroTwit using strictly a keyboard alone.) Simple works in mobile - it's why you don't see many complex Android or iOS apps - it has nothing to do with touch at all. Complex mobile apps, in fact, usually fail, and fail rather badly - KISS (Keep It Simple, Son) is the mantra for mobile developers. Desktop-targeted applications (regardless of what GUI they are targeting) can get rather busy - in many ways, they are EXPECTED to be *busy* (almost to the point of near-absurdity). How many light (utilitarian practically) desktop applications have been dismissed as being too LIGHT? (I can name two disc-burning applications that have been disparaged for committing just that "sin" - ImgBurn and CDBurnerXP - and both are freeware. I personally use ImgBurn because of the lack of frills - it does what it needs to and nothing else. It also works with any type of CD/DVD/BD drive Windows supports - if Windows sees it, ImgBurn can use it. If BD burners hit DVD burner prices - and they are nearly there now - ImgBurn is ready.) There may be a use for heavy - however, there is also a use for utilitarian.
Also, why must even a Win32 application rely on JUST a mouse and keyboard? Office 2013 certainly doesn't - like all versions of Office before it, it can be navigated using a keyboard alone (the Alt-key shortcuts are STILL usable - and throughout Office) as well (for the first time) as there is actual touch support (again, it's Office-wide) - supporting touch didn't kill mouse support any more than it killed keyboard support. ModernUI is a truly neutral UX - it works however you choose to. It LOOKS like an uncomplicated (in comparison to Win32) UX because it largely is - unlike Win32, it makes no assumptions. ModernUI - as a UI - is like ImgBurn - practically utilitarian. That's certainly different compared to Win32 - and it's typically been seen only in mobile (where utilitarian makes sense). That may be why there is that assumption - which I STILL call entirely wrong - that ModernUI is a touch-first (if not touch-only) UI; it's no more true of ModernUI than it is of Android.