Just by having alternative sources, developers might choose to only be available on those sources, whether it's for a better revenue cut or because of their principles.
See for example what's going on with exclusives for Epic store and Steam. Whether that's a good or bad thing is a different question.
(1) Apple, seeing drastically reduced app revenue, stops investing as much in the app store, dev tools, etc. Or increases their hardware prices;
(2) third party stores pay for exclusive apps;
(3) malware is installed by people with your contact info and communications;
(4) even if no intent to use, the malware risk is there and you're a few drunk clicks from losing your bank account.
#2 Frankly this is much like the situation we're in now. Only that we there are no other options. This eventuality is surely less bad than the current actuality.
#3 Malware is already installed by people you are in contact with. And nothing prevents a 3rd party store from having equally rigorous testing as Apple.
#4 You're a few drunk swipes away from emptying your credit card today.
If other people want to use other sources, that shouldn't affect you.