Britney Spears sells a lot of CDs too (or used to before everyone started filesharing). Quantity (as far as sales of action figures go) doesn't necessarily mean the movie the toys are based on was good. And there have been good ticket sales for the worst movies ever made. People also buy Transformers toys too and I'm sorry but those films sucked (IMO). There are many examples like that of modern day films in regard to ticket sales or merch sales.
Longevity can, however, indicate if something is truly 'quality'. If in 30 years there are people wild and raving about the PT (or Transformers) like we have with Star Wars, then we'll know. Somehow I doubt that will happen - the PT has already been widely classified as a disappointment by too many.
While there was excitement for it's release (Phantom Menace), the PT never inspired the widespread mania that the OT did (among almost everyone, not just sci-fi fans). There was a reason for that. Only among Star Wars fans was there a real excitement about the PT (aside from the general public curiosity that seemed to vanish after the release of episode one).
Also, most iconic characters from the franchise that you see in our culture (merchandising, etc) are from the OT, rather than the PT.
About the convention attendees being younger and "evidence" that this shows the PT was well liked- Two points to consider:
1. I'd wager that older people don't attend conventions as much as younger people do. That's what I've seen anyway. Lots more younger people at such events, but it seems that's because older people just aren't as interested in participating in activity that is generally considered to be more oriented towards kids and young adults - unless it's an adult themed event (like an antique fair or something). Younger people
do attend these things in larger numbers (and younger families who also may have kids young enough to enjoy the vacation spots nearby). Hence why you may see more people in their 30s than people in their 50s at Star Wars conventions.
2. As I said earlier, I think that the appeal of the PT was from the mystique created by the OT moreso than the PT being oh-so-great on it's own. Obviously they are the "back tory", but that doesn't mean you half ass it. IMO.
As a stand alone group of films the PT lacks, IMO. And that sucks because even though they were "backstory" there was SO much that could have been done. An amazing story could have been told that could have equaled or outdone the original trilogy.
Sean