Perhaps I am just fortunate to only play against players of my approximate skill level, then. Also, I don't play team games too often, so it is possible the ladder issues are more prevalent there.
While it is true that they removed mid-season demotions, this just means that league is no longer a direct indicator of MMR. For example, I could get placed in plat, lose every game that season, and end up playing versus silver/bronze players, and my league icon would still be plat. It is (arguably) not a flaw of the ladder system, as I am playing players of approximately my MMR. However, since I cannot get demoted, the silver/bronze players will say "why am I playing a plat player? The ladder must be broken!" When in reality, in the past the I would have been demoted, but am now unable to be.
I am in gold league, and I play diamonds sometimes. When I look at their match history, it is usually after a severe losing streak.
Like I said, though, I don't play team games often, so it is entirely possible that the ladder system there is messed up; I honestly don't know.
As for MMR decay, if you don't play for 2 weeks straight, your MMR decays slightly (the equivalent of losing 3 games or so). It really only becomes an issue, I suspect, if someone doesn't play for a whole season; their MMR get's reset, whereas it is unlikely that their skill has deteriorated completely.