Are you aware that on average retailers take 40-50% of your sales? The 30% that the App Store takes is a steal for developers.
Retailers? What decade are we in?
You're also totally ignoring other parts of that trade off. Being in the App Store means that you get yourself in front of millions of eyes that you aren't likely getting on your own website. The visibility is huge. It means tons more sales simply by people being able to find you easier and being in a more credible place.
This is mostly valid.
It also means Apple pays for the bandwidth. Downloads and updates take up a lot of bandwidth which costs money but developers don't have to foot the bill if they use the App Store.
Bandwidth is cheap. Way cheaper than that 30% cut.
Apple also deals with the payments meaning you don't have to pay a monthly fee plus a percentage of every sale you process. Additionally, setting this up securely and maintaining that security is not only a big time consuming process for developers but also a liability should anything ever happen to their customers information (and it can mean they're sued out of existence if something happens).
Again, what decade are we in? People use things like Square.
As I said above, 30% is far lower than what developers are typically use to. Anywhere from 10-20% less than everyone else takes.
I still don't know where you're getting this from. Are you still buying software in a retail store somewhere?