It would be a shame for man-millennia of work to be lost, but it would also be a shame for advancement to be stalled by these kinds of concerns. Experimenting with new approaches and ideas, direct competition between projects, excitement over new languages and frameworks. These are all factors that lead to advancing the state of the art.
It's worth being concerned about losing the lessons learned from Qt, but this is a kind of false dichotomy between the old and the new.
With that said, I don't think those lessons will be lost anyway. Qt can't (and shouldn't!) last forever or remain in its current form. C++, though it will live for a long time yet, will eventually fall out of favor, and large sections will eventually be purged from Qt as the project matures. The man-years that went into the portions that were, or some day will be, removed have taught Qt developers and users a great deal.
Qt developers and users will bring knowledge and wisdom to new projects that will compete with Qt (directly or indirectly). Some projects will survive and some won't. Some will be much the same as Qt, while some will be completely different from the start. Conrod, as it happens, is an approach to a different kind of GUI development (see this comment: https://news.ycombinator.com/item?id=8244224).
This, of course, doesn't include developers who do their own things without ever using Qt. Hopefully, they will encounter blog posts and articles about some of those lessons, as well as attracting developers who've learned some of these lessons.
This isn't a case of getting away with reinventing the industry. In this nascent stage, computer science, with all the renegade hackers and developers, is still a necessary occurrence when new languages and frameworks are invented.