Rank decay is pointless with how it's setup now. Regardless of your current rank your highest is taken into account meaning even if there was decay people would just let it decay since your highest is the only thing that matters, beforehand around s4 when current rating was a thing it would make more sense but its always a suggestion that isn't thought out enough.

Players sit on rank because there's nothing else to grind for, you dont get collars for x wins in a season or special top 10 rewards outside of the extra collars, if u dont want the weapons then u simply wont care and afk at diamond or safe plat and thats the bigger issue here than decay which is a plaster to a gun wound... At the end of the day this at most will affect those who want rank 1 / top 10 and will not help new players or lower skilled players.