Elo decay/inflation get really messy real quick.
Decay treats everyone as if they had the same resilience in acquired skills which is blatantly untrue. For me it takes around three games to be able to use the interface without feeling like I'm bending over backwards. I'm disoriented and uncomfortable. Then after that it'll take about another 20 games to get to where I was. Then I often experience the benefits of a 'perceptual reset' where I'm able to take all the things I knew before and improve on them by freeing my mind from the same old stale neural paths, becoming a better player than I ever was before. This process is not the same for everyone, many players seem to return and be exactly where they were withing five games. Some seem in a perpetual state of rust where they're never able to play quite enough to reach their old peak. There is no formula that predicts this unless the formula adapts based on patterns found at an individual level - which afaik is a helluva lot more difficult to make than the benefits of such a model is worth.
Elo inflation disallows comparisons between players across time. Even comparing the same player's position between one week and the next becomes a complex affair. StarcraftII did this when I was playing, and it was insanely frustrating, and resulted in inaccessible means of measurement. There were divisions, but you could be easily #1 in your division while being lower than #50 in a neighboring one within the same league. There were leagues (bronze, silver etc.), but in order to get to the next league once your MMR was settled you needed to perform better than 70% of the players in that division for an undefined period of times. Every adjustment to the system complicates it, removing it from the grasp of the common player, while also often creating further unforeseen problems. Eventually you get to where SCII was, where even an extensive googling won't let a player know wtf is going on.
Perhaps we should further split the scoreboards while periodically resetting ladder? We could have a hall of fame for lifetime scores, while also having the biannual(?) laddering. It's a tradeoff between the issues created by equalising elo (effectively turning every player on the ladder into a smurf), and the benefit of having continued activity incentivised rather than instilling the fear of losing elo on return. It wouldn't have to reset teams elo either.
In fact, you could go even further and separate elo as above while retaining old elo ladder separately. All-time elo, continuous elo, seasonal elo, teams elo. Everyone's happy (except the guy that has to code it). :)