You would certainly hope that UOR gains attendance overall after Epic's opening. It would be something of a disaster after all this investment if it just stayed flat overall.
It's all still up in the air where the attendance will come from in terms of people shifting from other parks versus the market growing. I do think there was perhaps a little too much fantasising from those who want to see Disney take a hit about how this would cause Magic Kingdom to drop down on the list of global attendance or how it would finally provide management with their comeuppance for under-investment, price rises, etc. As time has gone on, the capacity limits if nothing else have suggested that Epic's impact on Orlando's other parks can only be so great for the time being. My suspicion is more of those in the park on a given day will be people who would otherwise have been at one of Universal's other two parks than at any one of Disney's four, thus the hit to Disney will be less obvious. We shall see, though.