For what I need a i9 9900k (cant believe I'm saying this) is the cheaper option, plus I can actually get one.... I game I do nothing else so my opinion is going to be on gaming not "productivity" I can literally plonk the I9 straight in my existing kit and off I go, I don't need to worry about changing motherboards,memory or psu. Is the 3900x better than an I9 overall?? it probably is but the I9 is actually cheaper (worlds gone mad)if having the best was the choice for "overall" I wouldn't be buying either of them. AMD for me to choose everything need to be better and cheaper and a lot less niche. In niche I mean you look up AMD on the forums you get , you need this memory, this revision of bios, these drivers, it's hotter runs wore wattage etc etc, most people ergo "me" just want to plug sh^t in and start gaming, I don;t want to be spending an afternoon of valuable "free time" just trying to get the thing to boot up and or not make the lights flickers outside of an evening.
All I need to change is the cpu if I stay with intel, if I come away from intel, I definately need to change motherboard and processor, I "might" have to change the Cooling system and memory on top of that. You actually think the majority of "gamers" are on 1440p or 4k? I would be surprised if it's even 40% most likely around 20%