Why? I don't get the need for this.



Yes, this would work. Someone, make is so.

The reason for imposing a minimum gap (along with the other suggestions) is to add a degree of realism - WWI support trenches were rarely closer than 50yds to the front-line and often further away to minimise the impact of artillery barrages. The same was true of Japanese defences in WWII, and later on positions in Korea and Vietnam. However they were designed to allow troops to provide support and (relatively) easy access to the Front-lines

Attackers had very little in the way of targets to hit until they got close enough, while defenders had all sorts of advantages - Fire-lanes, pre-ranged kill-zones etc. In WWI the Germans perfected the ideas of defence in depth and reverse slope defences in the Hindenberg line, which they held with much reduced troop numbers until September 1918. The Japanese, and later the Vietnamese, took these kind of defences to new levels by having tunnels that allowed them to re-occupy ground that their opponents thought they had cleared.

WW I forces didn't have to face power armored chainsaw wielding supermen, or super fast bio-engineered killing machines, or any of the other 40k staples, all intent on jumping down into the trenchs.

Maybe if they had, they would have decided that positioning trenches closer together to more easily enable support to arrive to repel such attacks outweighed the disadvantage of increased vulnerability to shelling. Additionally, this is already modeled in game by barrage templates covering more targets when placed over trenches packed closer together!

"Realism" is a bad reason for imposing new rules with no actual benifit; applying WW I "realism" to a game set 40,000 years in the future is crazy.

I always understood that E:A was actually modelled on WWII though set in the future