The most important thing we wanted to achieve was to get a fresh version of QtWebkit. Which was not possible without upgrading to Qt5. We had plans to switch to QtWebEngine in future versions. But in current state, we're thinking about moving to Chromium.
WebEngine implements process separation and is based on Blink, whereas WebKit was in-process and e.g. allowed direct access to DOM from C++.
The job of moving PJS to WebEngine, if possible, when it happens will basically be the same effort as a rewrite. This release looks like a stopgap to get the best of the old WebKit line, but calling it 2.0 seems a bit shortsighted -- 2.0 should be kept for the WebEngine series if/when it happens.