The minimum range for suppression to kick in is simply too far.
Before we can judge whether it's too far, we first have to know how it's triggered and at what distance it's triggered. Right now it's all just speculation. All we currently know is that you're struggling to trigger it, which means that at the very least it's not very intuitive when you might expect to use suppression.
In VERY few scenarios would you ever be shooting at anyone beyond 100-120 meters max.
The point here isn't the exact distance. I was simply drawing a scenario where the engagement distance of a sniper rifle clearly exceeds that of an LMG. That doesn't change the fact that a sniper should have an advantage when they are at their intended engagement distance.
Laying down hundreds of rounds of LMG fire, as a game mechanic since you brought up that this game isn't realistic, should have an appreciable benefit to your team.
This is logic sounds a little odd to me. The benefit a weapon gives you is related to its ability to kill enemies within the intended engagement range. The LMG does not need suppression to exercise that benefit. It already has the advantage of being able to sustain fire for much longer than any other weapon class can.
If you catch an entire group of enemies off guard, you can maybe shoot 2 or 3 people with an assault rifle and you must then temporarily retreat to safely reload. In the mean time, the enemy has time to adjust to the situation. A machine gun on the other hand can keep firing until all the enemies are dead and then still have left to spare. Giving the enemy players little time to regroup and respond.
I don't see any issue with it applying an aiming detriment so that LMGs are a reasonable card to play against snipers sitting in a nest, as long as it's not "hundreds of meters away"
Suppression is not a mechanic that only affects sniper rifles. That's why you can logically conclude that the purpose isn't (just) to counter snipers. There are broader consequences that you must keep in mind, before you can properly evaluate whether it's a good idea. There's this common presupposition that snipers need a counter, without first exploring why they need the counter to begin with.
It would make more sense to first evaluate the effectiveness of the already available counters and the effectiveness of sniper rifles. There's no need to introduce additional counters, if you can either improve the effectiveness of available counters, or reduce the effectiveness of sniper rifles.