I just find it peculiar that Disney(and Universal of course too) encourages hopper features and pass it off as a "desirable" when they just admitted in classic indirect fashion that their parks are individually insufficient.
If the service and product were sound in each park in the first place then guests would not feel the urge to bounce around to another park or 2.......these parks should be self contained, fleshed out experiences.
I understand your point: If each park justified a full day, then park hopping would be superfluous. But I don't think people see it that way at all. Multiplex movie theaters show a dozen movies because people want variety. It's not an admission that none of their movies is good enough to satisfy everyone who would want to see a movie. It's a bow to choice and individuality.
I've never been to Tokyo (bucket list) so I wouldn't be able to compare. Envious. I am curious to know how many times you have been there. As I understand it, they have only two parks there, so I imagine it's rare for people to spend a week or more there. If I were to go there, I would be less inclined to park hop because each park would have so much of its own newness to me that wouldn't be tempted to go anywhere else for a day.
At WDW, you have lot's of people in the south who come for the weekend or a day, and people like me who stay for at least a week. Many people have also been there many many times. On our first trip, we had no trouble staying interested in each park for a full day. Now, I'd be very grumpy if I was stuck at any one of them for a full day. I like the variety, and I also like getting out of MK when the crowds get too insane. Disney is selling choice, not a grudging admission that their parks won't hold your interest for a full day. Also, I've never been to Universal, but as I understand it, you have to buy the equivalent of a park hopper if you want to do everything Harry Potter in one day. That's a far more obvious money play, assuming it's true.