This seems like an unnecessarily bold proclamation. The fact that Rory has kept collecting top tens (if only rarely being thick in the hunt) despite not really being in top form over the last three years indicates to me that he could win one almost by accident. If he keeps consistently being near the top of the leaderboard, he could catch a goofy US or British when the rest of the field struggles with a hard course or weather or something.
Day is still a bit more up in the air right now since he's in the midst of a 'swing change year,' so we'll have to see how he comes out of it. Wouldn't shock me at all if he returns to his old form in 2018, however.
Golfing fortunes can change in a heartbeat. At this time last year, nobody believed Sergio had the mental fortitude to win a major, and there were whispers that Spieth's Masters collapse had broken him for good as an elite player. Dustin Johnson looked to be penciled in for a monster season and then he slipped down the stairs in Augusta.
What counts as "dominant" in this post-Tiger world? Not sure what your timeframe was for Rory's future, but that's a 5-10 year stretch where he wins "a major or two and more tournaments," that's pretty darn good. That's close to a HOF resume itself, which would be an incredible second act to an already-superb career.