When they closed the registry on machine guns in 1986 there were/are 175,000 transferable machine guns registered. So if you want a run-of-the- mill WW2 Thompson, you're going to pay around 25K for one now. Meanwhile, Kahr will sell you a new semi-auto for under $2000. If they get rid of the NFA, Kahr will be able to produce a F/A gun for the same $2K retail. I'm pretty sure there will be less demand for a WW2 Thompson. those prices will drop. Stens, which cost about $100 to produce, won't fetch the $8-10K they cost now.
The collectables (Like early Colt Thoompsons) may hold their value because of their rarity.