If some of the components such as RAM were made replaceable, that wouldn't be the OEM's problem. They could still ship only a few configurations. When you decided the original RAM or storage isn't enough, you'd just ask your local shop to upgrade.

I'm not even asking for upgradeable CPUs or GPUs. Those could be a bit complicated on laptops due to chipset compatibility, power and thermal design issues, etc. But RAM is relatively cheap, can be made relatively easy to replace, and is a common thing to run low on. The same pretty much goes for storage. It seems weird and counter-productive to have components like that be soldered in and non-replaceable even by a professional.

I don't see why that's the case for Lenovo though (which I'm most familiar with - Thinkpads), since you have to order online. They could do batches of different configurations, it might add a small delay for particular configurations, but that would be ok. What they do now, mostly limiting larger RAM/storage configs to top end CPUs, seems like an upsell (though at least their storage is usually replaceable, and aftermarket offers better price/choice, though the spare small capacity device is a waste).

In Apple's case, it simply seems like an upsell. They must have enough volume to sell lower spec, large storage capacity Macbook Airs at stores, or online.

Most people buy low end machines from Apple. They just keep volume handy. Although recently when I went to Apple store they had no stock at all here in the UK of anything. Literally emptied out.

Yes, but how much does it cost Apple to put a larger volume in, compared to what they charge the consumer for that one feature? I have very often seen Apple users hampered by this, including losing data due to having to juggle external storage. Upsell.

Unfortunately, more often than not, the logic behind it is market segmentation, not technical or operational efficiency

I'm not sure which is the main thing and which is the pleasant side effect but I have to imagine both considerations enter their minds.

Tell that to European car manufacturers, almost no two cars are identical. I've heard that some models have more possible variations than there are v6 addresses.