I wish I could be optimistic here, but upgrading an older Mac can be difficult, expensive and honestly is usually not worth the time, money and effort once your Mac reaches a certain age — not to mention laptops, macs especially, generally offer limited upgradability.
Upgrading the battery, RAM and HD can be easy if you have the right tools and the right laptop model, but I think you're going to have a difficult time upgrading the motherboard or even finding a spare board for sell. I believe the MBA's generally have soldered on proprietary flash HD's, so I don't think you'll be able to upgrade that with the current motherboard. Also, I doubt the motherboard has a m2 NVMe slot, so if you ended up using an external adapter you're likely going to see diminished read/write speeds.
As a Mac/Linux user, I hate to say it but I've had much better luck upgrading junky PC laptops with Linux. 2 years ago, I spent a lot of money upgrading a 2009 MBP with a new battery, ram and SSD only to find that I can no longer upgrade the OS because Apple deems the hardware and the CPU is too old. With that said, I have a feeling that if you upgrade your MBA, you may find that you will no longer be able to upgrade the OS in 2-3 years.
I've cracked the Macbook Air open and actually have done a half-teardown of it - had a scare where I spilled water on the keyboard and needed to clean it up. The battery and SSD are easy enough to remove. And the adapter for the SSD isn't super expensive - it's to adapt M2s to fit the proprietary SSD slot -
https://www.amazon.com/Sintech-Adapter-Upgrade-2013-2015-MacBook/dp/B01CWWAENG.
They actually make proprietary SSDs that will fit but they are cost prohibitive at nearly twice the cost of a 'regular' M2 NVME SSD; and at least with the latter I can always use it with something else at a later time once I decide to ditch the MBA. The reads/writes people have clocked using the adapter + M2 SSDs are in the ballpark of 1300MB/s-1500MB/s read and write which is pretty darned good considering the most I'm getting now is between 300-500MB/s.
As far as MBAs having memory soldered on you are correct, which is why I am considering swapping out the entire logic board - the logic boards are what have the memory soldered on as well as the CPU but they are swappable so if you can find one in good/mint/excellent condition you can upgrade that way. But this, if anything, can be most cost prohibitive since the more desirable logic boards with 8gb of memory can run upwards of $200.
I actually posted over in Macrumors and started researching more there. Apparently, the 2017 MBAs are nearly identical as far as the inside components and how they fit. Several people have already stated they've swapped internals between models anywhere from 2013 to 2017, which gives me hope.
The amount of time it takes to do a teardown/rebuild isn't much in the grand scheme of things. Rather than buying a used 2017 MBA, I think this is a *very* viable solution to 'refurbish' an otherwise dated MBA to bring it closer up to spec in staying more "current" - but yea, the other consideration is how long of a "shelf life" the hardware has in keeping up with software.
Considering not that much has changed in terms of the internal configuration/layout of MBAs, it seems like they won't necessarily be "obsoleted" or "too old" to stay current in terms of OS updates if you take into consideration that there's virtually no difference in the internal layout between the 2013 and 2017 models.
After everything, this could end up costing around $450 or so but that's still a good cost savings over buying a new MBA or even preowned and comparable but not current MBA and where it likely won't be obsoleted any time soon unless Apple makes *drastic* changes to the MBA line. I would think the primary components that OS X would deem "obsolete" as far as hardware is concerned likely would be residing on the logic board (namely CPU and RAM but maybe I'm missing something?). On the other hand, I guess there's always the option of buying something "on sale" or "used in excellent condition" and then selling my current one to offset the cost. But this would be the case too if I were to swap components as well, so it's all relative.
