I've been thinking that the Thompson design could be updated and lighter if it used a bent flat ala an AK, with a pressed upper similar to the slide on a SIG. Years ago I was considering a Kahr, and I asked to speak to someone in the shop about the difference in receivers. I asked about the 'lightweight' (everything is relative I guess) aluminum version and asked the guy about them. He said if he were buying one he'd buy the steel version, which tells me something.
I think Thompsons look best SBR'd, and I love the originals, as do so many others, so I think people just consider it so classic that you shouldn't mess with it. I myself have mixed feelings. I think rethinking the Thompson design, perhaps even bullpupping it, makes sense. I don't like bubba guns however. I hope to be able to afford an AO SBR this year, and it will stay in wood, thank you.