It is generally assumed that such open source licenses are irrevocable, although there is some fringe debate on this matter, and some legitimate concern in the context of U.S. contract law (which is not applicable here).
However, the Qt situation does not involve the revocation (or threat of revocation) of a license. The Qt software uses a dual-licensing model, offering both proprietary and GPL licenses to the framework. The Qt company has announced that the availability of future versions under the GPL will be limited.
Because the old license remains intact, the open-source Qt community could fork Qt and maintain it themselves. There are many historical instances where a a company leading an open source project went against the interests of the community, and ended up getting forked. However, this is a highly undesirable scenario for all involved because this wastes/duplicates effort: Qt would no longer get bugfixes from the KDE community, and KDE would no longer get updates from Qt.
It may be worth pointing out that GNOME was started precisely because KDE is reliant on Qt politics, but back then Qt didn't even offer GPL versions. While the current news isn't good news, things are still much better than back in the 90s.