I don't think Plasma Active has ever been about "da apps". It also doesn't hurt that inherent to its design, you bank on the same demographic that Jolla and Ubuntu Touch do - Qt mobile apps. Any of those that ran on any of the above platforms, which would also run on iOS and Android, would
also run on Plasma Active. And all the desktop OSes as well.
Also, mobile is the perfect direction for a perceived "top heavy" desktop to move towards, because it requires you to simplify your UI to accommodate it.
The thing in the KDE world to understand is that QML changes everything. The Plasma widgets we have now were built on top of Qt4 series technologies and were often language hodge podges of Python, Ruby, Perl, C++, etc. They weren't hardware accelerated, and the standard toolkit was lacking compared to the QtQuick Controls of today.
And the entire QML thing is about fluidity. Whereas old Qt Widgets let you build wildly complex UIs that were pretty much static, QML is best suited to simple parts doing crazy transitions and fancy effects. Even if you convey the same complexity, the appearance difference can be enough to be intuitive for some where, say, kdenlive is not.
So I don't think Plasma Active is a "mistake". It is a great place to see the next geenration of applications for KDE in general come from, since they have to be focused. It is slow, and it is not going to see any market adoption any time soon, but trying to aggressively take the mobile market I feel is a mistake right now - it needs mindshare that requires Qt5 adoption which requires the KDE5 series to get into its stride first.