People who deeply care about free software would have never supported PlaneShift in the first place, at least as soon as they figured out that the claim "PlaneShift is an open source project" only applies to its source code, does not imply that any part of the project is free as in freedom, and really exists more because of some guy's idea that if you're going to make it open source, you'll be more likely to find people that will code for you for free.
I care deeply about open source and I supported PlaneShift.
I always saw the art asset/code divide like the OGL d20 system vs D&D proper. That seemed like a good idea becasue it advanced everyone's technology and still let Planeshift maintain its identity. I don't agree with the idea that just because Unreal allows source access for its commercial engine that it is an acceptable alternative, however. The improvements that may come out of Planeshift after a transition to UE4 would be useless to any other open source project. It requires a buy in to a rather clearly proprietary system. It is a lot more akin to D&D 4e's GSL where the improvements are useless outside of that ecosystem.
To me, switching to use a proprietary engine will cease what made PlaneShift so special. Epic and others are on the verge of exploiting open source development by hacking copyright, which is interesting in and of itself (I mean, that's what copyleft is too), but in the end such work is being done at the expense of creating an ecosystem of tools that would allow for truely unhindered creativity. I think projects like Planeshift as it exists now are essential to the future of the video games medium so that we can remove any possible legal pressure on creativity. The terms of the UE4 licence could change at any time and the projects that can assist in ensuring that there are well developed alternitives I really feel should do so.
I don't disagree that finding an alternitive engine is a prudent course of action, but as others have mentioned there are many to choose from that do not carry with them the legal and cultural ramifications of UE4. I'd personally put my had in the Gedot engine rink as it seems the most likely to provide a user friendly GDK.
In any case, I'd urge the development team to not give into the temptation of a shinnier gratis engine over a solid libre one.