I have no idea what ETFE has to do with a retractable roof decisions. If you built a car with a ETFE roof, would that be a replacement for sunroofs or convertibles? I don't think so.
I think most of the attraction it is to be in the "open air"...not just get some extra light in.
The ETFE is pretty cool...but so is open air when a retractable roof is open. Yeah, "pretty cool" is not necessary but "necessary" is not what anyone thinks about when make a roof retractable.
Sounds like it is all moot now and SU is not considering a retractable roof.