If the averaging mechanism (to the extent such has been described by the OP) isn't published and followed, people won't like it.

If it's (too) complex - Hilton's rolling 12-month elite qualification period confused many, and has been dropped - people won't like it.

Apart from the Choice benefits, pretty much all the other Plat+ benefits increase in value with more flying. (If you're not flying there isn't much value to free SDC, is there?) If you're not flying, you're not missing much.

As for how much flyer retention is improved by granting soft-landings (or long-duration qualification mechanisms) I bet there is a study (or five) done by Delta and/or NW that tested that very directly.