But that's one of the problems with PoS, it promotes hoarding and kills activity. Less activity leads to less adoption which leads to price decline.
Marketcap isn't everything, it's merely a snapshot of how much people are willing to pay for a coin and can change rather quickly.
It's too early to judge that PoS kills activity. Quite the opposite: In Nxt, numbers of transactions per day steadily increased since July until today (around 3k per day now, which is not a lot compared to bitcoin, but still).