I read a blog post about rating inflation, and think it's in interesting subject. It raises the question about whether the increase in ratings for newer shows means that anime is improving.
IMO anime has for sure improved, and I'm not just talking about animation. I think there is a level of sophistication in storytelling and characterization that continues to grow as time goes on. If you took the best anime series of 1985 and compared it to the best anime series of 2010 (for now let's just assume "best" means most popular, for lack of a better definition), I am pretty sure the majority of viewers, both today and timetraveled from 1985, would prefer the newer series.
But that isn't the whole story. For one, 95% of anime is shit anyway! This has always been true. for another, the main reason anime is improving is because it heavily borrows and refines from its predecessors. So even though Macross F is perhaps a better show than Macross SDF, I greatly enjoyed the latter more because I saw it first.
This explains the rating inflation a bit better. You will see a new anime watcher start with very high ratings for new shows and low ratings for old shows, because the new shows are "better" (i.e. superior iterations of the same formula). But as these users age and they begin to see the same shit over and over, they will start to rate the newer shows lower than they did at first.
Why, in that case, are new shows always the most popular? The answer is that anime has a massive turnover rate. Most people just stop watching anime sometime in their mid-20s, and the few that remain are drowned out by the vast majority of newcomers.