But, while lots of minor things change, the core of the champion is the same. Look at Sion. His main gameplay was rush at people with his shield and single target stun someone, then explode and kill an entire team. While the damage is less, his overall game play has stayed the same because now he just presses his ultimate, charges half way across the map, and rams into whatever he wants. Someone could play a champion and then items change/new champion comes out, and so they dont play that champion anymore..... etc. Why would you want to go from mastery 7 to mastery 6 then back to mastery 5 when it took possibly months just to get a single S- due to schedules and only being able to play a few games a week and then having to deal with trolls/playing against higher rankings (as silver i was for a while being matched vs 2 plats and a diamond every other game)?
Most do not want to, nor be required to, keep up with that on so many champions. I for one have not played a few of my mastery 6 or 7 supports for over a week at a time and then come back and go 2/2/32 (Nami gets my love lol), so for me, i would hate to have to get that mastery 7 again even though i know i could. I have already stated one of my main concerns about that (the getting the S ranks for it), but another one that is even more of an issue is the 600 Blue essences needed just to get that mastery 7 again, or 1100 to get from mastery 5 to 7. To get that, each 6300 champion is 325 essences. That is 4 of the highest priced champion shards available out of the.... 153? or so. Each shard chance is either a chest AND a key, or 1700 IP. I wont go into all the details about that, but im sure you can see the point being drawn.
With Ranked Queues in general, its a lot easier to do because it is only 1 game mode. One game needed every so often to keep rank. No cost except time to put into it. Will take roughly 20 minutes to 1 hour normally. The other can take days, weeks, months, or in some cases, years to get back due to how long they have to actually play and what events come up in real life.
i just do not see any way to justify the mastery decay for champions. Decent thought path, but I just do not see any way to justify it.