Because that isn't the only fix. It would simply make the deathloop even deadlier. Unfortunately the 6 men have to be accounted for when balancing the unit because removal would be detrimental because of the death loop.
Unless you can bolster durability in another way like damage modifiers or perhaps to think outside the box with something like increased range (longe range means further from the fight means enemy has to walk further under fire to frontally assault the maxim means they might actually eat dirt before being in grenade range) but imo the best way to make it less attractive to spam would be improving it, increasing damage AND build time, while also making cons more attractive so that they an fill in the holes that slow maxim production brimgs
What if we also increase cost to 300MP? But make normal arc like vickers and mg-42 have? Increased cost, increased time to build, tweake stats to make it worth to build.
Spammable only cheap units. If your unit expensive, but workable, it didn't able to spam. Better have 1 HMG that works, that have cheap HMG that don't work at all.