Or do they just kind of die suddenly? I ask because I picked up a used Comp M2, and the light is  invisible on the lower setting, yes I know those are the NV modes, but I don't see anything until the 6th setting, and it's incredibly faint, 7th setting is visible in low light, 8 and 9 are very visible. Haven't tried in bright sunlight yet. Wondering if the battery being low might explain it. I need to get some extra batteries anyway.

Somewhat, but lithium batteries usually discharge at 95% of their voltage until right near the end and they peter off real quick.

My Aimpoint T1 micro is still on its original CR2032 battery after about 3 years of constant on, and I can't tell any noticeable dimming of the dot.

So yes, most likely the battery needs to be replaced. If you have an M2 and it's been on for over half a year - especially if it's been on higher than the 'mid" setting (7 of 10 for the M2) they tell you it's battery life is rated at - it will seem to drain them faster and faster.

So if it's been on at setting 8 or 9 it will last maybe 6-9 months instead.

I saw some reverse engineered aimpoints you know generic models in afghanistan. I think they were made in Pakistan or China. It might be one of those.

Do ACOGs get dimmer as well?

That sounds about normal to me. I've owned/own 2 Comp M2's and a PRO and they are all as you described.

I can't comment on the battery life, I've never replaced one.

No.

I pretty much have to be in the complete dark to see the dot on 1.

Start seeing it at about 3 in a dimly lit room unless it's against a dark background.

Same here. I leave my T-1 on the "5" setting. It has been about a year and I really don't notice any dimming. I keep a spare battery in my grip JIC, the idea of a single coin battery lasting this long is still "witchcraft" (excellent engineering)to me.

I can't see shit on my T-1 until 5 or 6.

Yup, setting 6 on my T1 is where I start noticing things

In the dark you should be able to see it at 5