By choosing clones. Example: a few years ago, Adobe got greedier than usual and adopted the subscription model. That was enough to move me to switch from Photoshop to Pixelmator.
When some desirable app vacates the "main" store, similar apps will see an opportunity to add any missing features in pursuit of taking some share from them. Pixelmator got my long-term Adobe repurchasing revenue and has enjoyed it since.
However, I really do not believe that many will bail out of the main store for long. Yes, some will certainly go for a while... then miss the easy revenue from the most established store... then opt to have that revenue again instead of continuing to refuse it. If me, I offer my apps direct AND continue to offer them in the App Store too. Why? Because while I can likely make more money by direct sales on each app (even if I charge less than the App Store price by shaving something off of what was formerly Apple big cut), my website store won't have the reach of the App Store already on every iDevice in the world. So I want to be both THERE and HERE... and in any other stores where it makes sense for my app to be too... including "X apps for $10" bundles where I might make as little as a nickel on each sale.
Analogy: I make shoes. I could sell my shoes through Walmart. I could sell my shoes though my own website. I could work distribution deals with Target, Kohls, etc too. I very well maximize my profit for each pair of shoes sold for all who buy direct from my website... but I won't have the reach of Walmart, Target, Kohl's etc. By being in all places where shoe buyers may shop, I have a chance to sell all of them... even if those other channels will yield me less profit on the next transaction. So I want to be there too, as some profit is better than no profit. But I also want to sell direct, as maximum profit would have me pursuing the same profit objective as Walmart, Target & Kohls: maximize profit on each unit sold.
AFTER someone buys my app on the App Store or in another App Store, etc, maybe that customer will like my app enough to come direct to me for the next major revision and I can maximize my profit on that sale then? Apple or other distributor channel got a big bite of my creation's revenue on this transaction and I reached a brand new customer I may not have otherwise reached in a direct-marketing-only model. And now, I have a great opportunity to grow my relationship with them and perhaps they buy the next version direct from me next time.