re: SSD, yes there was a removable module but in 2018 Apple started using soldered NAND chips with the T2 as controller, same as MBP. For likely the same reasons I mentioned for MBP: a combination of physical size, performance, security, cost, reliability and margin.
The 2017 MBA and prior has always been a battery life champ compared to other Apple laptops. I love that I can replan the SSD on those models, the 2 and 4GB of RAM that was offered was ridiculous to begin with, 8 GB was decent and aligned the device for years of usability for what the device was designed for. I am not sure that there are many people using a MBA for editing HD/4K video or batch photo files. I use mine for photo editing, some quick videos, word processing, the the usual productivity daily tasks. I find 8GB quite usable, would 16GB be nice sure but again for my use case 8GB is decent, on the other hand SSD storage grows with what I am doing and I like the opportunity to expand that internal storage without carrying around external drives or depending on cloud storage for my immediate needs. I may not have the budget to get a computer with the top storage option however with the work I do I set small amounts aside that grows with my needs.
What is the point of getting a 8TB SSD internal storage when I am only using 1TB and may or mayn't ever require that extra space in the next couple years. I am stuck paying a premium now in the off chance that I may use it and if I do need it I will have to replace my entire computer or depend on external storage or cloud storage something I do not like doing while I travel.
Some options for upgradability would be nice to grow with the users needs vice asking them to make an immediate choice. I am even fine with all base Apple laptops coming with 8/16GB base RAM non-user replaceable and the SSD being user replaceable. A compromise, yes.
The T2 chip encrypts the SSD, if it was user replaceable there are options to make a clone of it or use migration assistant when upgrading. Plus there is FileVault that has been available for years. What is the purpose of having T2 hardware encryption and FileVault software encryption, what type of Top Secret work are you doing, seriously these just seem like a method to restrict the users more due to paranoia.