This is spot on.
The FB committee was charged with the task of keeping the FBs at the current performance level.
The threat was to combine FB and FA if the FBs continue to get faster.
I happen to think that the FB will continue to improve their lap times as the class matures. That is regardless of any engine improvements that may come along. I also think that FA will not improve much unless the engine rules are tweaked some more.
The other argument in favor of the restrictors is that it will help keep a parity between the engines we have now and newer engines that come down the road. I happen to think this is a very good argument in favor of restrictors. We need a supply of current engines but we don't want to "force" people to change engines every few years as the new engines out perform what we have in the class now.
So the long term issue will be to live with restrictors or be merged with FA.
Members of the committee spend a lot of time and money working up the proposal that is on the table now. The small restrictor for the "other engines" was simply that there was not enough data to set a restrictor size for those engines. But there is a procedure to do that. If you want a different restrictor size for you favorite engine you can do it.