Someone posted this https://macieira.org/~thiago/qt-stats/current/ in one of previous discussions. Commits from extrnal contributors are between 15-40% .

There are already a few forks from the past like CopperSpice, I think there is probably quite a good chance of a fork occuring if a major move happened.

All of them eventually die, because working for free on something like Qt never matches on feature parity, deployment targets or support levels Qt customers are used to.

I don't think it's really feasible to have a full fork. KDE people would probably be able to maintain the current features indefinitely, but probably not to undertake any big wave of innovation like Qt periodically has (the new 3d layer, QtQuick/QML, Qt4, etc).

Let's not be defeatists. If KDE forks Qt, certainly more contributors will join during the process.