I think you miss the point of a microkernel! The point is to keep all your "features" outside of it, and to use it only to implement the core set of functionality necessary to have secure shared access to the hardware. The general L4 concept has proven industry use, so there's no reason you can't take advantage of the already accomplished work on seL4 to bootstrap your own secure OS.
It's true that seL4 is not a magic safety or security bullet; in particular, their FAQ says that they haven't completed extending their proofs to DMA safety via VT-d or SystemMMU on ARM, so DMA-accessing services would have to be separately verified. And its particular feature set may not be appropriate for all situations. But if its API does work for you, it would be foolish not to at least consider using it.
It is really unreasonable to dismiss seL4 simply because it took them a lot of effort to create it. That effort is now done and can be re-used and magnified by further effort.
It's true that seL4 is not a magic safety or security bullet; in particular, their FAQ says that they haven't completed extending their proofs to DMA safety via VT-d or SystemMMU on ARM, so DMA-accessing services would have to be separately verified. And its particular feature set may not be appropriate for all situations. But if its API does work for you, it would be foolish not to at least consider using it.
It is really unreasonable to dismiss seL4 simply because it took them a lot of effort to create it. That effort is now done and can be re-used and magnified by further effort.