But the perception is going to be that D web frameworks are too fragile -- one miswritten handler, and your whole webserver dies.
Correction: "vibe.d frameworks" are fragile. This isn't D specific - my cgi.d is resilient to this (and more) and has been since 2008 since it uses a process pool. Simple solution that works very well.
Might not handle 10,000 concurrent connections... but you very rarely actually have to.