Not sure how that could change Jolla's answer, but Qt is not only GPLv3, but also LGPLv3 for most of its modules : https://www.qt.io/product/features#js-6-3
Some useful modules like QtCharts or Qt Wayland Compositor, Qt Quick WebGL would still be under GPLv3 only, so that doesn't change much the problem, and I don't know if you could have core modules as LGPLv3 allowing closed source app, and at the same time the GPLv3 additional module requiring open sourcing only for the app that uses them ?

Another point is that the Qt Project is currently releasing Qt6, which will break some API, whereas the Qt5 minor versions where intended to keep binary compatibility between them.
The first Qt6.0 will not be feature complete, but really soon probably all other platforms will switch to it, so that will widen the gap with sailfish and make it even harder to keep compatibility between platforms (you don't only need to not use or backport the new functionality, you also have to change the code to compile on both versions).
Also, the work to upgrade the full OS to a new major version would be harder than updating a minor one, which hasn't been done for several years.

My 2 cts...