Tim Bray articulates a couple ideas that seem right to me about RSS, though I might reach slightly different conclusions. I think mainstream RSS aggregation will happen on servers and the interface will be a browser—i.e., you'll go to a URL and get the feeds you want. This presents the opposite scaling problem of everyone pulling down the feeds individually; the server then has to serve a personalized interface to lots of people. But this is manageable. And has several advantages, from the user and business-model perspectives. If you're blogging, in addition to reading (and I'd like to think most people will be to a certain extent), that's the functionality that will be built into the browser.
Aside: Man, I wish we'd have had the resources to keep NewsBlogger going. It was a web-based RSS aggregator with blog posting interface we launched back in 2000 with Moreover. It was limited to Moreover's feeds at the time, but it didn't need to be.
Addendum: David Galbraith: "As the volume of RSS grows, I would rather use a web based interface to RSS in the same way that I now use Google to read newsgroups."