Open source does not mean that the intellectual property is free. There's a lot of good that comes from this, and it's not like those games are expensive.
Which, in the immediate future, makes me wonder less about the things that are going to be done in code, and more about the creation of new, free, visual and audio resources that make this work. That seems like quite a noble pursuit.
Correct. The license (at least, the one I read for Red Alert) is GPLv3 with some additional stuff. The additional stuff is mostly about not using EA trademarks in your version or showing any connection to EA itself. So it appears that a clean room asset swap would be allowed as long as it includes the title screen.
If you dig some experienced players comments who bought the "remastered" stuff, they are complaining that they "re-mastered" the exact same bugs that seriously annoyed the players too. They didn't fix the bugs.
With the power of OSS, every bug can be fixed.