If the company is big enough (e.g. not using whatsapp is not an option for a lot of people), or if the app comes from a state that can force you to install it by law, yes it can be.
As opposed to Apple being big enough that they can bully developers into both building for their platform and distributing through the channels they christen?
Don’t worry, you can bet Apple is going to make sideloading as awkward as possible within the bounds of the law. It’s not like Apple woke up one day and decided they don’t like money. It will only be useful for hobbyists.
