It look like a joke when unit cannot pass incompleted wall.
I agree, it’s annoying. Troops should also be able to access walls via gates and not just towers.
I take it you want to eliminate quick walling, one of the few things that I can do to save myself time while my allies don’t help me until 40 minutes in. 
Anyway, on topic, how do you display a wall segment going up without making it impassible?
they can, if you build a gate you can climb up the wall
in a game i used to play, the wall under construction would start off with a footprint that was passable by units, but upon completion it would not be passable on the footprint, Footprint = foundation of the building.
This has been a thing in the series since always, why would they change it now?
because its a new game, in a new Era, that means new mechanics, new strategies, keeping it the same they may as well called it AOE 2021.