What are you even talking about? Resolution doesn't affect how CPU dependent a game is, it affects GPU, unless you are running a REALLY old game from before tasks were split between the two (eg: games before 2006). In MH:World specifically, CPU usage is kinda high regardless of what you do because the game tracks EVERYTHING in the area at once with all the individual AIs, BUT almost no settings change the usage and the only setting that does have an impact on the CPU is actually better never turned on (literally no visible differences between on and off but HUGE performance hit, making it useless). Taking into account the tests I've done on the game (posted link to that thread in a previous post here), as long as the CPU is at least ~20% better than an FX6350 running at stock speeds and it's a Ryzen (which actually has proper cores rather than the FX CPUs which had cores but couldn't utilize them), you can run the game even on max settings, meaning anything above a Ryzen 1700 is more than enough for max settings provided you have a good CPU.Especially since you plan on playing at 720p, the game will be more cpu dependent as the RX580 can manage 1080p+.
@GameSystem If you don't care about futureproofing too much, go for 2700 (mainly suggest the 700 series because it's the best for overclock if you need more performance down the line) or similar, that way you will have some degree of futureproofing without driving the price up too high.
Also, about the built you posted, I highly suggest you DON'T go with 1050Ti, it's just an overrated card. MH:World is a game VERY heavy on particle affects, so unless you want the game to drop to 10FPS every time a Teostra does a Supernova and locking your input so you can't dodge it, then go ahead with the 1050Ti, otherwise go for the RX580 Finally, check what I said right above about the CPU, base game can run on R5 1500X, BUT Iceborne expansion will be even more CPU demanding (because of Hoarfrost Reach being a ridiculously massive map so even more things the CPU has to keep track of), so go for 2700 if you can get it cheap, otherwise 2600.