People have increasingly limited funds for vacation and less time off to spend it.
This is key.
One thing that hasn't been brought up is pricing tiers. One day at either Universal or Disney is incredibly expensive. You don't start seeing any value until you get to that 3rd or 4th day. So the fact that HP was drawing Disney families away for a day says a lot. The margin of profit Universal makes on those 1 day folks is higher than the multi-day people. And lets not forget that Universal is part of the Orlando passport program, where a person can pay about $300 and get 14 days of entry to both Universal parks, Sea World, Aquatica, and some other place I always forget.
There is a point when it Universal has enough stuff to do that a person planning a vacation has to decide, which park do we but the multi-day passes for? Sea World may not be the most popular destination these days, but a person who is willing to go there and buys that Orlando Passport can easily find 5 days of stuff to do across the multiple available destinations. Add Volcano Bay to the mix and you get much closer to a full week. Add a third gate to the mix, and a full week may not be enough.
Also missing from the discussion are the special events. Mickey has a Halloween Party, but at the end of the day what exact do you get and how much do you pay for it?
@$70 gets you:
5 hours (some of which cut into time you technically paid for already if you were there during the day)
Trick or Treat lines (which seem like a colossal waste of time & money... those end up being some amazingly expensive tootsie rolls)
A parade
A show
Fireworks (which you would have gotten on a normal night, but these are modified).
decorations
Some unique meet & greets
Shorter lines on some rides, longer lines on others
On the flipside, lets say you go to Halloween Horror Nights.
If you are already in the park for the day, you can "add-on": your ticket for about $50. If coming just for the Halloween event it runs closer to $100.
You get:
9 haunted houses. Arguably the best haunted houses in the US.
5 "scare zones" - interactive areas with scarers, themed decor, relative shows, etc.
2 stage shows (Bill & Ted / Jack the Clown)
I attended both events last year and the best thing I can say was it was "neat" to be in Magic Kingdom during the event. I think everyone should do it at least once. The parade was fun (even if the damn music was stuck in my head fort weeks... boo to you and you and you and you and you...), a M&G with Jack was fun, and the stage show was find of neat if for no other reason than to see the villains marching around. I can't imagine wanting to go back and spend that much money on it again though.
At Universal I felt like from the time the event started at 5pm until we left at midnight it was a constant stream of events to take in. The haunted houses were great, the Bill & Ted show as pretty funny (even if sophmoric), there were lots of unique beverages with flashy lights, and even Krusty the Clown's giant head lit up with blood red lights was pretty awesome. I enjoyed watching folks travel through the scare zones and some of the makeup in the Face-Off scare zone was top notch. I can see going back to this every single year if I had the ability to.
I know Universal competes with Star Wars weekends by having their Harry Potter celebration, and arguably, Universal goes further by having lots of actors come out and do Q&A panels, and maximizing the large themed areas they have. Presumably when the Star Wars area opens at DHS, Disney will be able to really ramp up their offerings as well.
This post is getting long (I have a tendency to do that), but the short of it is - it is not cost effective to spend multiple days at both parks. It can easily nearly double the cost of a families vacation. Both companies are priced in a way to encourage a family to spend a majority of their time at one place. Universal offers the free express passes while Disney offers their magic band + as well as other small perks for staying on site. But it is going to take more than perks to keep their edge that Disney has if Universal keeps aggressively expanding.
I actually spend the last 10 years exclusively going to Universal. I had gone to AK and Epcot back in 07' and then back to Epcot again for the Food and Wine festival last fall and spend another day going to Magic Kingdom for the first time to attend the Halloween party. That trip was a big change for me. For the first time in 10 years, IOA felt stale. The things I loved about it had slowly started eroding. What was once a vibrant area with tons of unique merchandise in Seussland has gone almost exclusively to shilling Thing 1 & Thing 2 merch. The Lost Continent I loved is a weird little shell of itself. Hogsmeade is impressive and all, but I it's hard to find the will power to shove my way into these tiny shops in hopes of seeing something interesting. It doesn't help that while think the HP movies are fun, I'm not exactly a huge fan. I appreciate Hogsmeade more for what it accomplished by being build as opposed to what it actually is. Jurassic Park always seemed to have broken stuff in the Discovery Center and a bunch of wasted space. It has a lot to do with the canceled pterodactyl ride I suppose, but it is just such a huge plot of land just for a single river ride and it feels misused. Toon Lagoon is just the worst. Show me a single kid that has gotten excited over Beetle Baily or Cathy. Ripsaw Falls has a great facade, but the theming on the ride itself is horrid. Just boring water ways with an occasional figure or sign. I have written multi paragraph posts on everything wrong with Toon Lagoon in the past. And then Marvel island is neat, but again, after exploring it for 10 years, I feel like I've seen it all. Apart from randomly running into a character, I don't have a lot to look forward to there these days.
On the other hand, USF has gone from being a park I used to not even buy a ticket for (10 years ago) to easily the best part of the trip. And with all the work being done at IOA now, it seems IOA is well on it's way to rekindling my interest.
This year we are taking the kids on their first Disney trip, with one day set aside for Grinchmas. It was hard to swallow that single day because it costs so much to get single day tickets. As Universal adds more, I think families who used to spend 1 day at Universal will start spending 3 or 4. At which point Disney's prices per day become hard to swallow. That will be the tipping point.