Ok, either you don't understand how pricing works in the MYW system (which I don't blame you for, it can be confusing), or I don't understand your question.
Are you asking why there isn't just one price to get into all 4 gates? There is. It's called a Park Hopper ticket. However, you have the option to also purchase a one park ticket if you only go to one park per day and want to save a few bucks. It's still one large resort, but you have options. If you only want to go to one park per day, you don't have to pay for the all access pass.
It's the same thing with the water parks. You can buy a ticket that includes the water parks, mini golf, etc. And that might appeal to people, since it's one resort, and you pay one price to see everything, but if you're not interested in water parks or mini golf, under this system, you don't have to pay for them. It's about options more than competition.
In terms of cannibalization, it's not really competition per se. When a new gate opens, the idea is that initially that gate will attract a lot of buzz and attendance will be high there. People will opt to spend more time there initially to check out the new attractions instead of the older parks, but eventually it will even out. The idea though is that overall attendance for the resort as a whole will go up, so even if MK initially saw a drop in attendance, it would come back up. This would happen because maybe I used to take 5 day vacations to WDW but with the new park, I now need 7 days to see everything. Or maybe the new E-tickets are so great, the 3 days of my vacation I used to spend at Universal will be cut down to 2 in favor of an extra day at WDW.
So opening a new park is NOT supposed to cannibalize business from the other parks in the long term, but if that park, or new offerings in the existing parks are insufficient to draw crowds, then it certainly can. Let's say I usually spent 5 days at WDW before DAK, I used to spend 2 days each in MK and EPCOT, and 1 day at DHS. Now that DAK is open, if it's a great park, that will make me extend to 6 days. But if it's a half day park, but if MK and EPCOT haven't added anything new in awhile, I can just decide I've seen them enough and can cut EPCOT down to 1.5 days, and see DAK in .5 days. That's an example of DAK cannibalizing the other parks. I did go to DAK, but I didn't extend my trip at all. Two ways to combat this are to make it so that DAK (or any new gate ) is a full or multi day park with lots to see and things I'll want to experience over and over, and to add attractions to the existing parks so that it will always seem new and I'll continue to feel I need multiple days there, thus prompting me to have to extend my trip to see it all.
HTH
