Well, as nice as that sounds on paper, there's a reason we use reverse-proxies in the first place. It's because it turns out that opening anything up to the web is a whole can of worms of exploits, malformed requests, differently complying useragents, tons of standard, nonstandard, and "crazy off the wall bullshit" headers, and a LOT more.
Greenfield writing a new safe reverse proxy that works within a magnitude of performance of something like nginx is a monumental effort. Not to mention the chicken-and-egg problem of how nobody will trust it until it has significant usage which it won't get until some people trust it.