I really think Epic will be the tipping point for more guests where they go from Disney vacation with a few days at Universal, to Universal vacation with a few days at Disney. Universal's hotel pricing is much better than Disney in terms of bang for the buck, and the third theme park will make it even more appealing to stay at Universal for the duration of your vacation. As it stands, I think age is a big factor for those decisions, with the parents of young kids and seniors making the Disney choice, parents of older kids and younger adults making the Universal choice.
Disney will always be the top draw-- Universal will continue to erode Disney's market share though, unless Disney really starts investing in new attractions. With all the things converging right now-- Disney in flux with management & possibly ownership/fighting with the district authorities & state/cutting spending company wide to make it more attractive as a takeover, and Universal aggressively investing by the middle of the next decade the whole balance of Orlando tourism could be very different.