You've asked what benefit there would be in making a free Qt replacement. One benefit is that it would become possible to include KDE in a free operating system and have it actually work. KDE is free software, and could legally be included in a free operating system. But including it wouldn't be useful, since it would not be able to run, Qt being missing. Qt is not free software, because it has unacceptable restrictions.
Free software means that users have a certain set of freedoms.
It is definitely not worth the pain. The purpose of making a free replacement for Qt is to make programs like KDE usable for free operating systems. If it achieves this job, whether it is behind in development is not important. It would be natural for people to focus on writing the features needed for important applications like KDE, and leave the other features of Qt for later. But you are right that this is a big job.