Well.. i'm not going to start an argument about the technical side, since you're just guessing as much as I am. But UE4 is definitely not made for huge map sizes especially not for multiplayer. From a thread I found on the UE Forums:
" By default size of the world is limited to ~500k units (WORLD_MAX constant in sources). You can still place objects outside of these limits, however you will start experience unstable physics simulation on objects that are too far away from world origin. So it's not recommended to place objects which simulate physics that far, but you can still place visuals (ex. skyboxes usually are much bigger than 500k). Right now engine will stop physics simulation for actors which are outside of world bounds(WORLD_MAX), you can disable this behavior in WorldSettings->Enable World Bounds Check. "
The same post says there is a tool for "world origin rebasing" which is supposed to help with maps bigger than this but it also explicitally states this tool is still experimental, so probably too unstable to base a serious development endeavour on, and also not suitable for multiplayer games.
Another argument is the sheer size of the file which a WWIIOL sized map with the levels of detail shown in the screenshots would have to be.
On another note.. I can't really confirm your performance issues with the view distance in WWIIOL and I disagree that the huge view distance is not important and should be sacrificed. The View distance limitations in Squad and Post Scriptum are very much noticable, especially in hilly maps in Squad. HMG's completely lack the utility they would realistically have in such a terrain just because you can only see half as much from an elevated position as you could in real life. Especially Close Air Support would have a WAY harder time making out targets than they already have. Honestly, the rendering distance for infantry should be increased. You cannot see moving infantry in a plane from a distance where you should realistically still be able to make them out.