The problem of not having a broad view is not just an IETF problem. Look at the evolution of the sciences and the value of cross-fertilization for example. I am pessimistic that the IETF could cross-fertilize well without having everyone at plenaries... but I'm also pessimistic about it happening even with the big plenaries. The best cross-fertilization seems to happen inside participating vendors, and then get expressed in the IETF.
Scott