Feel like I got the "there's more things to do in the US that theme parks" bit right at least, the US is a really big Country after all and I was lucky to be in the northeast for most of my life where you can get to a lot of interesting things within a day's car trip (historical sites, parks, cities, etc).
Also talking from personal experience. Even when my family went to the parks, we would usually go every other year and alternate with other stuff (different beaches, trips to other places in the US, etc), feel other people do the same. Besides, Universal and Seaworld have been traditionally something you tack on when you do something else in Florida either as a a second part of the trip or as something you do on the first/last day before you move on (which is one of the big reasons why EU is being built, so Universal can use it as a way to permanently get out of that perception), so when people are doing things that don't involve going to Florida, the Orlando theme park industry will collectively suffer.