How bad that would be remains to be seen. It highly depends on how much external work from external contributors is going into qt. if most of the work comes from internal employees, then the BSD fork that was auto-created will stagnate while they can add all the cool stuff to their commercial offering.

They could then proceed to make their maintained version as incompatible as possible to the open source version.

The outcome will likely be a very two very different versions of a toolkit.

One based on BSD qt, maintained by the KDE project, more and more losing its cross-platform features (why would KDE care about platforms it doesn't run on).

The other will be the commercial continuation of old qt. Incompatible with the open source version, but keeping its cross-platform features and seeing heavy development and feature additions.

Of course this is just speculation, but it is at least conceivable.