Mortars aren't for entrenched positions, and that's the fundamental flaw in their game design. Mortars are for breaking up concentrated attacks in open terrain where the shrapnel can fly everywhere. The chances of landing a mortar shell in a trench is quite difficult, and will at best only affect a few soldiers since trenches are built with curves etc. The only time you would expect mortars to affect an entrenched enemy is if you've got an unlimited number of shells, and all the time in the world.
On the contrary. Trenches offer protection from direct fire weapons.
Mortars are indirect fire weapons specifically designed to hit target behind obstacles. Trench warfare was there bread and butter.
WWI was a war of artillery after all.
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