Unless you literally had a single thread of execution running on each core, you would still have to worry about all those things you mentioned. Presumably, Cloudflare's software is too complex to make that feasible. So while it makes sense to bypass the kernel for the biggest bottleneck in their system, processing IP traffic, it wouldn't make sense to give up the convenience the kernel provides for things like scheduling, disk drivers, etc. Plus, security would obviously be a concern. Running complex internet-facing software in ring 0 on the bare metal all the time is like riding a motorcycle naked. Sure, you might be able to go a bit faster, but if things go wrong, there is literally no protection.