I don't believe that the ATF, as a regulatory agency, as the statutory authority to add weapons to those required to be registered. Letting an agency make decisions that big would seem to be an improper delegation of legislative authority, and therefore a separation of powers problem.
And it was the National Firearms Act of 1934, duly passed by Congress, that put Title II firearms on the list of things that needed to be registered. NFA, by the definition of that statute, doesn't include semi-automatic weapons.
I suppose they could argue that all semi-automatics are "readily convertible" to fully automatic, but that would be a stretch, and not likely to hold up in court.
Still, the underlying question seems to be: if Congress added semi-automatics to the NFA, would you continue to buy them?
My answer is: Yep, assuming that they don't also raise the tax, since it was originally intended to be a prohibitive tax and just hasn't increased with inflation. If the tax was $2000 instead of $200, then no.
Aaron