It might have to do with the relative minority of System fans compared to Bionicle on the site. Since Bionicle is kind of what kicked this whole thing off, not many of those fans were also hardcore System fans.
I might also say that a good majority of the people you see making frequent topics these days are a lot older (high school to college age). The plain and simple truth is that not too many people are interested in showing off a LEGO set they just bought, especially when it's not that hard for people to find pictures or reviews of the exact same set online at any time.
To that end, it might seem like people don't buy LEGO anymore, but I don't really think that's the case. It's just that you don't really see people saying they did.
I wouldn't say that. People still like Bionicle, and I'd wager a lot are into Bionicle exactly as much as they used to be. The only reason it might seem like we don't like it as much is, again, because we don't have anything to post about, nothing new to discuss. There aren't sets anymore, so what else is there to do but MOC and make creative works? And frankly, I'd say a majority of the content on this site are those things.
I have to respectfully disagree with this sentiment. There's nothing inherently childish with maintaining appreciation for something from your childhood, and certainly nothing bad about still holding love in your heart for something that has long since past in popularity. Where that becomes an issue is when those old interests become debilitating obsessions, but for most of us they serve only to inspire us.
And I might also add that Transformers as a franchise, while still immensely popular, is no where near as popular as it was back in the 80s. So to consider Transformers a "modern" franchise that people get into because they don't want to live in the past is a bit off the mark with Transformers history.
"To be concerned about being grown up, to admire the grown up because it is grown up, to blush at the suspicion of being childish; these things are the marks of childhood and adolescence. And in childhood and adolescence they are, in moderation, healthy symptoms. Young things ought to want to grow. But to carry on into middle life or even into early manhood this concern about being adult is a mark of really arrested development. When I was ten, I read fairy tales in secret and would have been ashamed if I had been found doing so. Now that I am fifty I read them openly. When I became a man I put away childish things, including the fear of childishness and the desire to be very grown up.”
- C.S. Lewis