I don’t think it’s a fair comparison. And right now I can think of 2 reasons:
1. Unless your app is targeting solely European users, publishing it outside the AppStore would mean you’re annihilating a large share of your potential user base. Because if you’re a user based outside Europe, no matter how much you like the app, there’s no way for you to get it. On the Mac, every user can side load, so every user can still get it even if it’s published outside the AppStore (albeit with the eventual inconvenience of not having it on the AppStore)
2. Even then, developers aren’t dumb, and they know they’ll face some resistance from users to get the app from outside the AppStore, even if you’re only targeting the European public, so many developers will prefer the existing solution of publishing it on the AppStore, like the vast majority still prefer to publish on the Google PlayStore despite having the option to distribute it elsewhere. Will this happen 100% of the time? Most likely not, but I don’t think it will have that much of an impact. It will be up to the developers to decide whether having a smaller user base is worth the extra income per app sold. Right now on the Mac, the resistance from users to download something from outside the AppStore is close to zero, because it has always been like that, and people are used to it being that way.
Bonus: the other day I’ve decided to check whether the apps I have on my iPhone are available on android or not. They were either available on the PlayStore or not available on Android at all. So I’m not very concerned about these changes. Most likely close to zero practical impact. The upside is being able to get apps Apple decided to veto for God knows why, if you really want them. Will I be sideloading? Most likely not, but I’m happy for those that want to.