Android users spending less money doesn't mean no money. There is money to be had, it is just less than iPhone users. And devs did try their own App Store and continue to. Epic for example has their own for Fortnite.

But what happened is that no one wanted to jump through the hoops of enabling third party app installs so most had to bend the knee and come back to the Play Store. If Apple is required to allow the same, I see the exact same thing happening. The majority of users won't want to jump through hoops and won't do it. Some devs will try to force it, but the loss of income will make them come back.