No you clearly don’t get it. The parties offer something that people want. A special event that differs from the standard offering that you can get with a standard ticket. Enough people want that special event that there is a high demand market for it, and the pricing is reasonable so it keeps selling out. By the very definition of offering something special that you don’t get with a standard ticket on a standard night, you are going to change what happens at the park. It is also the only thing that is relevant. WDW is a business, finding things that its customers want, at a price they are willing to pay, is the only thing that it relevant.
Second, there is no needless or adverse effect on non-party goers. The whole point of a special event, is that you are paying for something that is different and not available with a standard ticket/night. So yes, people who buy those tickets are getting something different than those that didn't, or to put it another way, people who elect NOT to pay for those special events will NOT get what people who do elect to pay get. Just like people who pay more for park hoppers get to enjoy multiple parks, while people who paid for a single park ticket don't. You get what you pay for, and by your choice you get a different experience.
Third WDW does satisfy both its party and non-party going guests. If you don't want to go the party, that's your choice (complete aside i highly recommend going to each at least once.) You can still go to MK that day and stay the entire day, from opening, to when the park closes that day for general public guests. OR you can go to another park that day, and decide to visit MK when there isn't a party scheduled, OR choose to visit WDW at a time when there are no parties scheduled at all, OR you can go to the party. WDW makes all those choices available to all its customers. But just because some people elect to pay more to spend more time at the park after it closes for the day to the general public, and YOU don't doesn't mean that your experience is negatively effected. If i elect to pay more for first class, it doesn't mean your experience is negatively effected bc you choice to pay for coach. Or if I go to Sky Zone and pay for my kids to jump for 2 hours, but you chose to only pay for an hour, its not negatively effecting your experience when they kick your kid out of the trampolines after their hour is up.