It's only implemented in the top 10% of the player base, which at that point I would hope you are playing regularly.
top 10% is still like masters and up. even at that level the gameplay really isn't competitive enough to NEED the decay system. having afk accounts sitting in masters doesn't affect anything.
last season i played (less than because i decayed like 300 overall) 1 game per week and still performed above my level and had a solid win ratio, i literally just do not have the time or want to play a game every day.
its a stuff move for people who want to play competitive casually if that makes sense. yeah i want to grind out ranks, but i also dont want to and shouldn't be forced to dedicate unnecessary time every day or else face losing an insane amount of sr