LFS will never be open, at least in the near future. There have been very heated discussions on this, which I'm sure you're aware of. Its The Team's choice at the end of the day.
To be honest, I'm partially in agreement with Scawen / Victor / Eric on keeping LFS closed. As much as I love Open / Libre software, some projects just dont benefit in the long term. Whilst Apache maybe the greatest bit of HTTPD software of all time, a massive amount of shit gets dumped into the trunk, which the release manager then has to clean out before release time. Its likely that the same would happen with LFS. Plus if LFS was openned then Scawen would have to spend his time documenting the whole thing. No progress made. Just look at Open Office; about 6 people understand it as a whole.
I'm afraid to say the people who want the ability to create custom tracks an vehicles for LFS is tiny. Something else to note is that most of the games you've listed were in an almost complete stage. LFS is not. As Scawen said at the meet, the tools change often, create in compatibilities, etc. The community couldnt cope. Wouldnt you be a bit pissed if you updated a tool and your track which you Just Finished(TM) was screwed again?
Things never work out that way. Sometimes it would be better to fork, or even branch an open source project, but how many damn bits of software just start from scratch? Humans like to do their own things, they're not inherently collaborative. For instance Micheal Meeks who works on OpenOffice said he once wrote his own spreadsheet in java to get out of doing the actual accounts data he needed it to do, and also because it would be cool.
First group....that conflicts with your point above. You've already shown they aren't the first, and they definately wouldn't be the first open source game which you can mod. I'm clearly not understanding this point...
I'd argue that whilst Microsoft products maybe the most widely deployed consumer systems, they definately arent the most "compatible", in the traditional sense. And to be honest, theres no reason why you can't or shouldn't run Open or Libre software on a closed platform..