Putting on my fan hat for a moment here...
What are you talking about? (your post is v long and reads like an agenda.)
Gatecon 2008 was essentially RDA's first foray back into conventions since the 80's. This is not a 'small' deal. He was also at the 2016 Gatecon - is he not 'big name' enough for you? Was Gatecon supposed to get Kurt Russell and James Spader? (If so, there wouldn't have been any other guests b/c those two would've taken all of the money.)
RDA isn't 'throwing' the party. Gatecon is run by fans. If RDA is asked and he accepts, then he'll attend, but there isn't a guarantee on any of the guests until the convention actually begins and they show up.
The reason Gatecon tends to attract the 'same old guests' over and over is because those folks live in Vancouver. They're already in town, so it's much easier for them to swing by the convention, even if they can't commit to the full weekend. Alex Zahara has done this - he was working one year, but he stopped by anyway just to say 'hi'. A lot of the 'regular' guests love coming to Gatecon to talk with fans they've seen before and meet new ones. Sure, these five or six guests have been every year, but not every fan can attend every year, so there are always new fans in attendance who haven't seen/met/gotten a photo with certain guests who would like to. If Gatecon doesn't invite those guests, those fans might not get that opportunity.
You don't speak for all fans. Whether or not you liked SGU or the tie-in novels, there are fans who did. Skipping those guests is doing a disservice to the people who worked on those properties, and also to the fans who did enjoy them who may want to meet some of these folks. There's no reason to skip them, since many of them did other things that people might have also enjoyed.
Bottom line - Gatecon isn't Creation. Creation paid for the 'official' Stargate convention lic (and it ain't cheap), so Creation gets 'first dibs' on pretty much everything. This is also why Gatecon didn't have some of the principal actors - Tapping, Shanks, etc - during some years, b/c they *couldn't*. Creation has rules about when those lic'd under a property can attend conventions that aren't Creation events, which makes it difficult for other cons, especially smaller, fan-run ones, to invite those guests. Creation Entertainment is a business. Running conventions and other events is the ONLY thing they do. Gatecon is a business, but it's a side-gig for the folks who put it together. It's not their only job, so it's smaller.
With that said: Is there a limit to the number of guests? Technically 'no', aside from how much money is available. However, also, technically 'yes', because Gatecon had 52 guests last year and it was too many. It was great, but it was also a burden. Trying to coordinate that many high-profile people at the same time is a logistical pain in the mikta. (Full disclosure, I'm also a Gatecon volunteer and worked the 2016 con.) While it's awesome to have a huge line-up of guests, it also makes the convention more difficult to coordinate and run, which means far more things have to overlap and the volunteer need starts to multiply. So, a smaller guest list is actually better for the convention overall.
To your suggestion about running multiple panels at the same time: No. As a fan, at Gatecon, I wouldn't want that. I like that the panels are one-at-a-time. I also like it better when panels don't overlap other things like photograph or autograph sessions. Single panels means that, as a fan, you don't have to decide which panel you have to skip when two of your faves are up at the same time on different panels. That's a horrible decision to have to make and I'm glad Gatecon doesn't make the fans do that.
TL;DR: Gatecon is a smaller, fan-run convention that has no intention of matching 'the big guys' and the fans of the con itself like it that way. New guests are always welcome and encouraged, but the Gatecon family also likes to see old friends as well. XD