While the contours of how sideloading is going to unravel is unknown at this time, it is possible that Apple may still use some form of app notarization (perhaps an API for 3rd party stores and require that the stores use those APIs -- similar to their stance on webkit).
Then, similar to the Netherlands case where dating apps were allowed alternatives to InAppPayments, but charged 27% commission, Apple might charge a discounted commission (relative to the 30%) for sideloaded apps.
They will look to get their cut.
I don't like it, but that seems to be where this is headed.