Well, FP+ debacle notwithstanding, attendance WILL go up at Universal no matter what. The Harry Potter expansion in USF opens this summer. The Hogwart's Express will connect the existing IoA Hogsmeade area with the new London/Diagon Alley area in USF. So not only will Universal be selling a lot of tickets, they'll be selling a lot of 2-Park tickets.
They are doing major PR. There are already videos out, one with one of the actors that played the Weasley twins, a big HP weekend convention thing later this month, and a big "streaming" event on the 23rd, where folks suspect they will show more footage and perhaps even announce the opening date. No doubt so they can start filling those hotel rooms.
Disney's attendance may go up or may go down, but Universals will go up this year guaranteed.
Personally, I think that if Disney's attendance suffers, it will be NEXT year because THIS year everyone's going to go, hoping for the best, find out what it's like and not reschedule. That's what just happened to us.