I know the original context of this thread was geared towards M16s, but I'm actually in this situation.
The FRT's are a game changer. I've never been an "AR guy" and could/would never consider a transferable M16. However, I now have an SSAR and will be getting the 10/22 FRT. Probably SBRing to make it a quasi AM180. These will be excellent toys for dumping ammo and have a real cool factor.
That being said, I'm still getting back into the subgun game. I sold my M11 several years ago, and recently realized how much I missed it. Cleaning, modding, reloading, show and tell, subgun matches, the overall simplicity and the mechanics of them. All of this combined is what draws me back to a subgun. Is there a 9mm FRT platform that would allow me to do that? I'm not sure. Like I said, I'm not an AR guy so an AR9 has no appeal to me. I have a semi Scorpion that would be fun with an FRT, but it'd be a challenge to tame the recoil with the blow back short stroke. MP5? Perhaps.
Ultimately there's something about an old open bolt subgun that is very appealing to me. But again, this is me and you can do you
But back to the AR/M16 question- I suspect that lightning links and sears will drop in price slightly (much like registered Uzi bolts), but registered receivers will be ok and probably be unaffected by the FRTs much like others on the thread have said.