I have noticed it too. Lately my feed is full of resteem from some curation communities and only a handful from the people I have chosen to follow. As you said maybe it is because I followed a lot when Ι first got here and then neglecting to do so. It looks like only a few stay for a long time...
Mine's pretty much the same, mostly photography curation groups and a few people that still post a lot. While there doesn't seem to be that many that have stuck around, I imagine quite a few will be back once the price of STEEM goes up. Until then I guess we just have to make new friends. Now if only everyone would delegate their SP before they went inactive...
Yes I have thought of this wasted SP. It is too bad but once people turn inactive you can't find them to propose an alternative. Hopefuly some of them will come back!