Pretty interesting discussion. I haven't gone through all the repiles but I know enough to comment. Unfortunately i generally disagree with the OP and would add that I hope he doesn't feel ganged up on, but it's a bit too late for that
From what I understand of the many interviews I've read and heard with the Anet big wigs (Jeff Strain, etc.), GW was from the very beginning designed to be different from the typical MMO. They intended almost from the get-go to offer a MMO-style game that did not require a monthly fee and the franchise would be kept alive by selling optional, stand-alone expansions.
This is very unique and so far, I have to see it has benefited consumers very well. The barrier to entry is low (I myself was introduced to GW thru Factions first) and players can get involved with as much or as little as they please. It is a very flexible system and Anet has admitted that no, they make less money per player than MMOs like WoW. To balance that, they probably have to spend a lot less money maintaining their servers. The world is not persistent, so there's less need to staff gamemasters because everything is instanced.
I digress. As far as I'm concerned, Anet IS adding new content with the release of new chapters every 6 months. Every time a new chapter comes out, refinements are added to the overall core game. The ingenuity behind this business model is that (ideally), they bring in new players with each new release as well as cultivate renewed interest with their established fan base.
I don't see how suddenly charging players a monthly fee would help Anet's cause. Think of what would happen if one day, all your favourite websites that you surf for free asked that you pay them a monthly fee to view their content. We'd see these website lose serious traffic to their pages.
The OP suggests adding a monthly fee to further develop existing content (i.e. Prophecies), but I have to wonder about the magnitude of these improvements. I mean, for $15/month, they had BETTER add some pretty impressive things to the game because it perfectly fine before as a "free" game. Furthermore, how do they market this change so they retain existing customers and entice new players to the existing 3 GW games? With a monthly fee added, I will expect fundamental gameplay enhancements. And if that happens, why not just release an all-out sequel instead?
I get the sense the OP loves GW enough to want it to last indefinitely... but nothing lasts forever. Even a game like WoW will eventually fizzle out. Time will go on, technology will advance and new games will be created to raise the bar for MMOs.
It doesn't make sense to ask people to start paying for something they've been getting for free. And if you do, you better add something that makes it worthwhile. And if you do that, why not just go all out (for example, make Guild Wars 2)?