Now you might say that much of today's software is too complex to describe in a man page --- but IMHO - that's the bigger problem. If people write complex monolithic bloat, writing pretty documentation for it is the least of our problems.
I wouldn't say that today's software is too complex for man pages but instead man pages have never really been ideal for the tasks for which they're used. Software has always been complex. Man pages might have been appropriate for some short window of time but technology quickly left them behind.
Man pages do not have an effective system of hyperlinking, indexing, or even searching. They were meant to be read on a teletype or printed on paper. For documentation any more complex than instructions on how to use console commands they are completely inadequate. Even for looking up instructions on console commands they're less than adequate because there's no sort of authoritative hierarchy, if you don't look up the exact right term man won't point you to the correct documentation (or best guesses).
Besides man being inadequate it is difficult to write proper man pages. This is just adding insult to injury as it makes it less likely that developers will write even bad documentation.
Of existing documentation systems I'd most like to see GNU Info become the primary documentation mechanism for FOSS. It solves most of man's problems without introducing its own new ones. Even GNU Info isn't perfect and could use some improvements.
I don't disagree with the idea that FOSS desperately needs some reliable offline documentation. This idea might require that FOSS distributions themselves maintain their own documentation. The Arch wiki for instance is fantastic, it's some of the best Linux/Unix documentation around. While the Wiki is great it would be really nice to see this information turned into texinfo/manpage/whatever files so everyone could have good references and not need access to the internet.