Good luck _selling_ that phone to the hundreds of millions of users you lost immediately overnight, to users already perfectly happy with the phone they have. Your customers aren't going to pickup the tab just because you didn't like the distribution terms you had already willingly agreed to on iOS/Google app stores, and its a big tab. A new phone isn't cheap for most. I am also completely ignoring the fact building a competitive phone is likely really hard and Epic have no track record there.
I'd love to be proved wrong of course - who doesn't like a cool new tech platform succeeding? Being realistic though, there isn't exactly a long list of new mobile hardware/OS platforms launching at the scale and success Epic would need to replace lost iOS/Android revenue. In fact, no one has ever managed to do that really! That Epic have already partnered with Nvidia to use browser streaming tech to get Fortnite back onto iOS suggest similar thinking to me. Epic's own lawyers are making similar arguments right now too.
Imagine if every railway company had to build it own tracks, or every airline needed its own airports. Conceptually, app stores on mobile platforms are increasingly similar. It's not practical to keep building more airports and more track networks, especially when in this case the airport or network is a handset the customer has to buy. The railway example is one Courts had to deal with in early 20th century in a lot of countries, same too for shipping ports.