I admit I don't have MP experience so this may be silly, but don't you want to research the 38/40 infantry techs every time (given that they improve your army without additional IC cost). That would significantly reduce the opportunity cost, wouldn't it?
In older versions of the game, I might not even be doing that in a timely fashion. It might be more important to get better ART in the field. Plenty of ART techs that upgrade the firepower of your artillery without an IC cost.
There's also the issue of what rules you are operating under. Before we restricted ahead of time research, there was little reason to waste time with infantry weapons research. You really needed to get those 1944 planes and 1943 tanks researched ASAP, plus relevant doctrines.
Under the new version of the game, I'm not sure what to do anymore...
Well since artillery is useless now, might as well upgrade the inf equip. At least to inf2. Inf 3 comes a bit too late if you are playing with tech rules/don't have bonuses and has 80% reliability instead of the 90% reliability of inf 2 and 1 so inf 3 is questionable but I almost always try to do inf 2 now.
You're not the first person to suggest this. I still need to test some things out. If everyone is going to go ORG wall with their infantry and ignore ART altogether, I wonder if wasting time on Kit II makes sense in the long run. Yes, the firepower upgrade is nice, but if we are going to create massive walls of ORG, there's an argument to be made that you'd rather have another X number of men equipped with weapons than have slightly better ones in the hands of existing formations.
And if you're talking about loss of production efficiency, would a scenario where you make a new line with inf2/3 and prioritize armoured divisions and special forces for it, leaving regular and garrison infantry on inf1?
I've considered this as well. I know players who do it. I was going to do that in my last game, but it ended before this plan could be implemented.
I always feel like a loser, though, when I add infantry equipment production lines later in the game. I feel like I'm letting down the troops by just giving them rifles instead of tanks and planes.
(Offer good only for rifles, though, because Secret Master can't be bothered to make even more tanks and planes.)