Fortnite's revenue is about $1.5 million a day on iOS and is pretty consistently one of the top 10 earners on iOS each day. So in the long run it more than likely matters to them. That being said, if you're Apple you're probably trying to figure this out before more companies decide to jump ship or attempt to reroute. Like you said, if they don't figure out something they may start opening the door for other companies like Samsung or Microsoft.
Apple and Google both take a 30% cut from micro-transactions, which was easy to find information, but it's not clear if MS / Sony / Steam / Nintendo do the same. All listed platforms take a solid 30% of actual game sales, and Nintendo apparently used to take 35% in the Wii days but adjusted it to be in line with other platforms.
Microtransactions on Android follow the same structure in that they are required to use the Google infrastructure for payment in order to be on the Play store. Since Android supports third party stores, it's more symbolic to remove it from the Play store, but third party stores don't receive nearly the same amount of traffic. Epic is accusing Google of undermining third parties through all of the security warnings about installing third party software or side-loading applications, but it's always seemed reasonable to me since I'm sure there is plenty of malware parading as a store front.
MS and Sony probably get a similar cut for microtransactions, but if they targeted all platforms then no one could play at all. A court ruling against Apple, though, could set precedent for other platforms pretty quickly.