Xbox takes 30% of the full game purchase but not that specific percentage for DLC and other microtransactions.
I assume all platforms take % cuts it is nowhere near as high for IAP. If I'm wrong, please show me where that's indicated.
Epic is known for their 12% cut in their own game store which is a lot lower than the competition - and developers are flocking to their service as a result. Their model does work.
Developers aren't flocking to the Epic store. By far the majority are launching on Steam. Only a few huge publishers are big enough to be able to push their users onto the Epic store and all of them suffer backlash as a result.