This has been an interesting discussion. I'm not convinced that runDisney has reached a saturation point yet. Yes, there were days when events would sell out in days if not hours. However, since those days, they have doubled the number of race weekends offered in Anaheim and added another race weekend to Orlando. Inaugural race weekends have sold out insanely fast (Inaugural Avengers, Star Wars 1/2
Disneyland), but the subsequent years have not sold out immediately. This is not a bad thing for anyone. In the immediate sellout days, you had a lot fewer options if you wanted to try for the Coast to Coast or just wanted to participate in a runDisney event. Now, you have 4 1/2 marathon distance events offered on both coasts not to mention marathon weekend at WDW.
Case in point: In 2011, my little sister ran Disneyland 1/2 with me. She was going to sign up with a friend to run TinkerBell except it was sold out. This past October, she signed up to run last weekend's Star Wars 1/2 with me even though the race had been on sale for months. She only missed out on the Rebel Challenge which she was considering running, but not completely sure about. According to the announcers last weekend, Star Wars 10K had 13,000 runners and the 1/2 had 16,000 runners. Those do not sound like numbers that runDisney is at all concerned about. In fact, Star Wars had a lot more corrals than ever before. So even if the race isn't selling out, those are probably still numbers that help especially with the sheer volume of runners
inside the parks that weekend wearing shirts, medals, etc.
I understand why some runners have chosen not to continue running Tinker Bell because it moved to Mother's Day weekend, but I also understand why Disney chose to move Tinker Bell to that weekend instead of keeping Tink in January and moving Star Wars to Mother's Day.
As for Avengers, it's a little bit of a different animal. Disney bought Marvel back in 2009. And even then it would be a few more years before Marvel characters even made an appearance on Disney owned property (Avengers monorail wrap in 2012 maybe or Thor meet and greet in 2013?). Furthermore, Star Wars has been a part of Disneyland for just over 30 years now since Star Tours opened in 1987. Star Wars was a part of Disney with theme park rides, stage shows, Star Wars Weekends, Disney/Star Wars mashup merchandise, character appearances in the parks long before Disney actually bought Lucasfilm in 2012.
I also wonder if Star Wars essentially pulls runners away from Avengers. At least speaking for myself, I could have registered for the Inaugural Avengers. But I had heard rumors of Star Wars coming. I already knew that I could only go for one race registration that year and decided that a Star Wars rumor was better than a guaranteed Avengers. I also read complaints from many runners who would have skipped Avengers if they had known Star Wars was coming.
How many of us are in a similar position? Because of lodging, travel, and ticket costs, we have to choose one or maybe two race weekends. In that situation, we either choose our favorite theme (Star Wars for me) or the weekend that best works with our life schedule including family times, professional responsibilities, etc. I don't think runDisney or the parks on both coasts are too concerned about those numbers right now. If all races start to see Avengers style numbers, then they're seeing a substantial decline and may indeed have a problem on their hands. But for now, I think it's much more a case of supply and demand are relatively balanced right now, which is good for everyone. I've talked to a few people that would have signed up for a race but learned about it days after it went on sale and thus days after it sold out. Now runners don't have to sign up immediately and runDisney has more inventory available.