I agree. The rule has always been "once a machinegun, always a machinegun".
They've been doing that for years. I got a re-import A1 kit five years (or more?) ago. They were pretty common at the time, I think even Sportsman's Guide had some. The kits had the barrels, one or more parts of the select-fire control system, and lower receiver removed. All the other original Colt parts where there. Some kits were re-barreled, some not; some with FSB & gas tube, etc mounted, some not, depending on which re-importers you looked at.
Even that took some doing. For a while ATF held that possession of any M16 parts constituted "constructive intent" to build a full auto weapon and thus was a violation. That meant owing an M16 bolt carrier was illegal, even though installing an M16 carrier in an AR-15 won't make it a full auto weapon.
It took a relaxation of that very narrow definition of "constructive intent" to make those parts kits legal.
The Un arms control treaty also creates problems with that in terms of preventing signatory countries from exporting military small arms, even as receivers parts kits.
A concern here in the US, even absent ratification the UN treaty is the ability of a particular administration to narrowly define other statutes or impose export and import controls via executive order that may preclude re-importing M16 parts kits without the lower receiver.
I do hope the M9s will be surplussed to CMP, since the precedent has been set with the 1911.
I agree that this more likely with the current administration than it would have been under the previous administration.