Welcome, are you staying around long? (honestly asking btw)
Depends on how much stuff there is to leak. ;D
W3D doesn't interest me anymore... you should contact the RenX team and see if you can get your hands on their code. This place seriously needs some UDK love. Otherwise it'll be dead on arrival.
Port stuff to UDK, publish some fancy ads, attract new folks and so on
People need to stop assuming that moving to a new engine fixes jack. Maybe no one noticed, but RenX belly flopped so hard after the first few weeks of its release. If a game is good people will play it despite the graphics, original counter strike, mine craft, etc stand testament to that. The issue most W3D games have is we have shitty advertising so no one hears about them.
There is also a down side to having a large community off the bat:
If your game releases with a major bug, now everyone plays your game for the first time and is imminently exposed to that, which potentially turns massive numbers of people off to ever trying it again.
All engines have pros and cons, some cons you don't see until you start developing on them. If these teams were slapping out a new game every year it'd make sense to switch engines, but when it takes years to complete a game there will always be something better to move to before you complete, its just better to complete one thing at a time or else you get nothing done.
ECW actually got written consent from EA, I assume Reborn and AR did as well.
And your cons about no source code is pretty much completely off, we have about 60% of the engine source in TT 4.2, and hook the rest, if we need more we simply reverse engineer it. Saberhawk is currently working the engine up to dx11 in TT 5.0.
A pro to W3D is that its scripting language is in C++ which in comparison to Unity (which uses javascript and C#) is vastly faster.
I will admit, I was rather surprised that RenX managed to actually make vehicle handling worse than stock renegade, if that's what you mean by "with better physics."
You should cross unity off if you want Multiplayer and players able to exit and enter vehicles, because Unity does some odd shit with how player control of network objects works. If you're the creator of the object your computer is what does the updates. This works fine if its just you playing, but if someone else gets in a vehicle your computer created, every key press they make has to go to your computer, where it controls the vehicle, and then sends the update back, which makes for painful amounts of lag. Although it is possible that they have fixed it, I haven't touched unity in 3 years now, but I rather doubt they did. I feel if ECW were to move off W3D I'd go for cry engine, as it'd likely support large open environments better than what UE3 did.