Generally, this is a very good idea, and we have considered it in the past several times. Genode OS Framework has a very clean and though out design and also has excellent documentation.
But as you said, the effort for porting is very significant. There are several paths we could achieve that, but all are of the scale of “several man-years of work”. I hope we will have enough resources for that, one day.
BTW, one of the key differentiating factor of Qubes OS versus various microkernel-based systems, is ability to run unmodified applications (Linux, Windows). It comes at a big price in terms of resources, of course, but still that’s something we are want to keep. That means, even under Genode OS Framework, we will likely still use virtual machines, even if for less parts than we use them right now. On the other hand, you can run much smaller VMs for specific tasks under Xen - namely, you can use unikernels. And there are a few implemented specifically for Qubes OS (look for qubes-mirage-firewall, or qubes-mirage-ssh-agent). This is a way to bring some values from microkernel-based systems to Qubes OS as it exists right now.