The rift system was made to address what became a glaring problem with public quests in Warhammer. Public quests were a fantastic idea, but after only a few weeks it became hard to find anyone willing to go do them. The ones closest to towns could see some action, but anything that was a bit out of the way was a ghost town shortly after launch. Rifts didn't eliminate this problem by any means, but because people didn't want their quest hubs/towns taken over it encouraged people to fight back against invasions. Even higher ups would help out in younger areas.
It is actually the biggest problem with non-instanced dungeons too. They are fun for a short time and then they become wasted content. Places like Sol Ro, Guk, and Seb were all awesome content when they were relevant. Then they became museums that only saw use when someone was feeling nostalgic with a Alt. Compare it to dungeons like Wailing Caverns in WoW which still get run dozens of times a day because of the dungeon finder/cross server system. That is almost 9 year old content STILL getting used. Say what you will about instancing, and WoW, but that is pretty damn efficient work.
I think that is why the open dungeon model is likely dead and gone. No one can really make a good argument to justify it. Why make something that will be a ghost town in a year when you can make something that will see constant activity for the better part of a decade? Unfortunately "but I prefer it!" doesn't count for anything when they are looking at the numbers.
