Human activity is ruining ocean habitats and virtually eliminating some species, but the jellyfish seem to be thriving, and as you say, are helping kill off other species. Not a hopeful picture at all.
When I was a teenager, I had one close encounter with jellyfish at the Jersey shore. It was late August and the water was quite warm. I was headed out beyond the breakers to where I could actually swim and when I got there, I noticed all these little things brushing me. I looked closer and they were jellyfish about 1-2 inches in size. They didn't sting me, but the water was thick with them and they were getting inside my swimsuit. I completely freaked out and raced back to shore, showered and changed into dry clothes, and never went into the Atlantic Ocean again.