Applications I've written using Qt twenty years ago still run with Qt 4.4 on all relevant platforms (Windows, Linux, even MacOS under Darwin) without any need for a change. As long as the old Qt version compiles on a specific OS version, there is no need to change and no one has to spend time for maintenance of the Qt version.
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If a customer pays you to do the maintenance and update, I hope you're doing it.
But regardless, this is why integration testing is important.
"What will break if I update to Qt-latest?" being one click away.
I go the other way round. The project should not depend on stuff only available with the new new version of the library. Even if I use Qt 5.4 or 5.9 on some projects most of it would still run on Qt 4.4. And these are not only open source spare time projects.