I've been part of a full-freedom multiverse RP for several years now. Although I cannot speak specifically to PW's situation, I can explain what keeps us all consistently invested, even though interest rises and falls. 

TL;DR: We made the whole thing ourselves, becoming invested in it and developing a sense of responsibility. It is this sense of responsibility that holds us together through ups and downs. Self-control holds the RP together and self-starting distributes the burden of plot and world development across the whole group.
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[b]Investment:[/b] Myriad Reality was collaborative in all respects—everyone created important aspects of the multiverse on their own, getting some direction from the wiser members of the group but with free rein on their creativity. This occurred organically. As we filled the world with more and more of our brainchildren, MR became something we could not let go of, at least in spirit. As we became invested in "our place", we became responsible for it, too, so even when business and humdrum pulled us away, we always came back for more, powering through disinterest because MR was important to us.

[b]Responsibility:[/b] Creating and upholding a free-form multiverse required significant self-control and self-starting. With no GMs, one asks, how do you prevent violations and contradictions? How does anybody get going in a story if there's nobody to lead it?

The former required two things: common courtesy and wisdom. We all gave each other room to shine and tried to be sensible in the way we wrote so we wouldn't push anybody out. We work things out OOC to prevent such contradictions, because we've already designed a framework

The latter required a self-starting spirit. I have observed, over time, that some RPers would rather follow a GM's plot like sheep. This isn't necessarily a bad thing, but we all have a desire for "easy entertainment" and if that desire is funnelled into an RP then it leads to passivity and lack of effort. In a free world, the more people acting like this, the more burden is placed on the few that love starting things. In Myriad Reality, we all start things and finish them—as much as we enjoy playing within the plots of others, we also create our own and draw everyone in. Nobody is passive for long. We wouldn't enjoy it, anyway.
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It is not immediately obvious to me how to apply these principles to a roleplay with GMs and stricter control. My kneejerk reaction is that the solid framework of your creation and of the Star Wars franchise will help prevent contradictions. But it will also heavily restrict the freedom that makes collaborative worlds so magical and enticing. If I am not mistaken, it will feel more like a normal (but bigger) roleplay than a collaborative community. The major burden will be on the GMs, not on the people, and this could be very draining, especially as interest fluctuates.

I shy away from providing any more advice because I have never GMed a roleplay in my life. But the last paragraph is just what seems most sensible to me, drawn from my experiences.