First of all, Qt 5.2 is seeing plenty of new APIs and improvement for authors of QWidget-based applications. Here's a random smattering of concrete, technological examples:
* KDE upstreamed tons and tons of little widget features from its kdeui library into Qt, e.g. more complete keyboard navigation and title support for QMenu, tab bar hiding for QTabBar, default vs. active shortcuts in QAction, clear buttons in QLineEdit, URL drops in QComboBox, place holder texts in QTextEdit, ... there are several dozen improvements to widgets, I don't know where to stop. KPrintDialog features in QPrintDialog is pretty big. QColorDialog, QInputDialog, ...
* KDE upstreamed lots of things it used to do via KGlobalSettings and KStyle into QStyle and Qt's platform plugin system, which means pure-Qt QWidget apps now integrate a lot better with KDE and potentially other target platforms.
* KDE upstreamed its KStandardDirs APIs by adding and extending QStandardPaths, which allows applications authors to more easily deal in standard locations on various desktop platforms.
* KDE upstreamed lots of work on Qt's MIME type system.
* KDE upstreamed its X11 session management handling.
* Lots of stuff in QDesktopServices, QCommandLineParser, QLocale ...
So ok, that should put the "no improvements" spiel to rest; you're not really aware of what's been going on. But even if this wasn't so - why would QWidget be bad to use just because it's not making huge changes?
Let's be clear: Qt 5 swapped out the entire backend underneath QWidget by porting everything to QPA, and it just keeps working. That didn't take no effort. That's called commitment.
* KDE upstreamed tons and tons of little widget features from its kdeui library into Qt, e.g. more complete keyboard navigation and title support for QMenu, tab bar hiding for QTabBar, default vs. active shortcuts in QAction, clear buttons in QLineEdit, URL drops in QComboBox, place holder texts in QTextEdit, ... there are several dozen improvements to widgets, I don't know where to stop. KPrintDialog features in QPrintDialog is pretty big. QColorDialog, QInputDialog, ...
* KDE upstreamed lots of things it used to do via KGlobalSettings and KStyle into QStyle and Qt's platform plugin system, which means pure-Qt QWidget apps now integrate a lot better with KDE and potentially other target platforms.
* KDE upstreamed its KStandardDirs APIs by adding and extending QStandardPaths, which allows applications authors to more easily deal in standard locations on various desktop platforms.
* KDE upstreamed lots of work on Qt's MIME type system.
* KDE upstreamed its X11 session management handling.
* Lots of stuff in QDesktopServices, QCommandLineParser, QLocale ...
So ok, that should put the "no improvements" spiel to rest; you're not really aware of what's been going on. But even if this wasn't so - why would QWidget be bad to use just because it's not making huge changes?
Let's be clear: Qt 5 swapped out the entire backend underneath QWidget by porting everything to QPA, and it just keeps working. That didn't take no effort. That's called commitment.
Edit: Good and pertinent comment by someone else: https://news.ycombinator.com/item?id=6566635