The ISU saw too many skaters turning professional from that big circuit of events, so as you mentioned, they took over 10 events in the ‘98/‘99 season, becoming open events for pros and amateurs. Some of them were interesting, such as the very first one- The Grand Slam of Skating which started with rosters of 8, then cut to 4, and then a final head-to-head like in tennis. But the recently turned pro/still amateur skaters tended to dominate this stuff even if their skating wasn’t matching it. Find Lu Chen from this competition and you will scratch your head over how she beat some of the pro skaters. And Butyrskaya, who was en route to her World Championship, competed in many of them on top of the GP schedule.
Skaters like Nikodinov, who at that point hadn’t even been to a Worlds yet, were being sent to the previously very prestigious Challenge of Champions. Diana Poth was randomly added to the Canadian Open either that season or the next because she was training around there (in Detroit) IIRC.
The World Pros became the Hallmark Pros (Dick Button sold it off), and moved away from Landover/DC. I was at the final one in Columbus in late 2002, and even with a ladies field of Sato, Slutskaya (current World Champ), Hughes (current Olympic Champ), and Butyrskaya, they were giving away tickets to the event near the arena that day to try to fill up the embarrassingly empty stands.
I think SLC 2002 didn’t help the cause and many people will tell you that’s what did pro skating in. But for me, I have a feeling the ISU knew what they were doing in the fall of 1998 and they really did want to get rid of as much pro opportunity as possible. By the very next season, only a few events remained.