That’s a poor example with 4 infantry vs 1 plane. First, it’d be more like 3 infantry vs 1 plane for more equal IPC cost. Secondly, this is pointless when you’re not considering naval nations. I’ve already said a million times that infantry is easily the most valuable unit for Germany and Russia who have to prosecute large land wars. That’s never been the argument. For some reason you still argue that infantry is the most valuable for everyone, when this is most certainly not true.
Even if you just count the number of infantry for US/UK/Japan, you do not build more numerical infantry than other units, much less spend more IPCS on them. This is because you have to use transports to get land troops anywhere, so you are building infantry in equal numbers with tanks. You honestly would load transports with 2 infantry rather than 1 infantry 1 tank? In that case then yes infantry would be more valuable if people actually did that, but that’s not the case. You don’t build primarily infantry as naval nations, ever, unless you’re fighting a last ditch effort and you’re going to lose anyways. You’re busy matching 1 infantry with 1 tank, and tanks cost more, so tanks are the more valuable, and if you had the opportunity you would put 2 tanks in the transport since you need to muscle your way through and space is limited so it’s not a question of which unit is cheap fodder more like you need quality units in the transports.
Infantry is obviously most valuable if you don’t have to use transports and your deployment limits are high compared to your income (Germany/Russia). For other nations you’re busy building equal numbers of infantry as tanks, and you’re busy constructing navy too, so infantry can’t be considered the most valuable for those nations. If you’re doing amphibious assaults you don’t try to attack with mass infantry because you need good muscle, and because space is limited on transports you need all the muscle you can pack in.