The 1080 is absolutely fine for 1440p, in my opinion. Mankind Divided is incredibly hard to run, but so is XCOM 2, a game that not everyone cares about. A different example: try running GTA V at max settings—8xMSAA, Ultra grass, extended draw distance, etc.—and you'll need 1080 SLI to hit a consistent 60 FPS in grassy sections at 1440p. That's in a game where SLI and Crossfire scales incredibly well. Considering certain missions online encourage you to outrun the cops by climbing grassy mountains, you'll be taking a huge hit if you expect to crank settings.
Many games were clearly not developed with current GPU's in mind. They included certain options just for the top ~1% of PC enthusiasts, for future generations of GPU's, or for those who don't mind playing at 30FPS. Who could play Crysis 3 at max settings when it was launched? Practically no one. Only those with 3-way 680 SLI or 3-way Crossfire 7970's at 1080p could expect that. And what did they get? Ever-so-slightly cleaner lines. It starts becoming about diminishing returns by that stage. You can turn settings down in Mankind Divided and it will still look great. It's a slow-paced stealth game so doesn't benefit that much from 144hz monitors or really high frame rates. I think you should just swallow your hopes of cranking every setting in every game—many of which are superfluous or force diminishing returns—and be happy with a single GTX 1080.