As soon as WDW opens 2 or 3 other rides that are as amazing as RotR, this problem will resolve itself. Until then? I'm not sure. I was lucky and was there on Dec 13 and 14, before the park-opening, join-a-BG system started, and getting there early was basically a guaranteed BG.
There must be a better way to handle it than what's going on right now, with the BGs filling up in 60 seconds. But somehow I don't think allowing people who aren't in the park to join BGs would alleviate this situation. I'm picturing BGs gone within 5 seconds or less if you didn't have to be there.
Also, as someone who was privileged to be able to ride twice on consecutive days, I kinda doubt that restricting people to riding only once in a 10-day period, for example, would do much to change things. It's not like there're thousands of people who did this. Of course I don't have the actual data to back up my surmise, so it's just a guess. What I'm getting at is that limiting the number of times you could ride in a period of X days would probably still not solve this problem.
This difficulty involves a ride with a lot of segments, any one of which could break down and a heckuva lot of people who're psyched to experience it. As word gets out more, even more people will want to experience it. WDW must be working on doing the best they can with this. When SWGE opened, it seemed like maybe it wasn't going to be the grand success WDW had counted on. But now? Now it seems like SWGE is a success way beyond anything they expected.
I think the Star Wars hotel is also going to be a gigantic success, despite its probably very steep price.
ETA: Stayed tuned for special ticketed SWGE events. It seems inevitable.