Until we have a way to extend seL4 or something like it to full multiprocessor operation (without the multiple kernels with separate resources limitation that is currently the only way to use multiple processors with seL4) I'd disagree that we can build general-purpose OSes with verification. Our techniques for verifying concurrent programs are still very primitive and cumbersome, and I don't think many would take an OS where processes can't use multiple hardware threads seriously.

Also, seL4 (being a microkernel) leaves out a huge swath of kernel-level facilities that need to be implemented with the same standard of verification (resource management, network stack, drivers, etc.). Running on a verified microkernel provides a great foundation, but these still add a ton of code that needs to be verified. Plus the concurrency problem will strike again at this level.