Not specifically targeting you with my reply but this was a similar justification the SMAC referenced when approving the floor drop.
And every (nearly every) new build now has a dropped floor.
Same goes for the offset bushings - now every new build or replaced UCA gets the offset bushings.
About the only allowance in the past 2-3 years (from my recollection) that hasn't appeared to attain some form of critical mass in terms of a "must have" update is the allowance of the 1.6L front sway bar on NA1.8s and the Cusco LDS in the 1.6s. If you are racing a 1.6 competitively, you've got a Mazdacomp and judiciously maintain it and have a spare on the shelf should it fail. For the sway bar, we already have enough roll stiffness up front.
The position of "not being forced" to do something has never been a solid foundation to approve an allowance from my experience yet the pursuit of one from a safety perspective (see floor drop) seems to have a higher probability of being approved by the CRB.
There are many rules in SM that don't require a modification or upgrade to be installed. Anything that is widely available and not terribly expensive that improves safety or longevity or cost/time/effort ought to be approved IMO unless there are other negative consequences. In terms of the subframe braces, a case could be made that they be mandatory after a given date.