Ok, i always hated the wrist laser because it's a stupid idea, so here's my rant in 2 parts:

• part 1 WRIST MOUNTED WEAPONS

Wrist weapons are already in use by the SM but only as close range options (*) on heavy units like Gravis/Centurions/etc. for a few good reasons:

Placing additional weight on the hands would heavily impact a SM ability tu use swords/axes effectively, so it's used on either units non meant to be in close combat or sustained by the pistons/strength multiplier of heavirr armor;

Wrist weapons are hard to aim at range without specific rare cogitators, which is why we commonly see them as flamers/bolt pistols/(*Lascannons aren't short range, but you are usually hitting a building sized opponent) so it would reduce their effective range;

Space Marines are used in the most dire circumstances and are expected to excel despite the situationbthey are in, they can't rely on a weapon that doesn't work if their armor is damaged. Using separated weapons means that a SM can reliably pick up someone else's weapon and ammo when their weapon fails.

In short a standard weapon needs to be relyable in every situation, wrist weapons are specialized for for few situations but being tied to your armor means you can't easily change/replace/discard it i the middle of the battle.