I think in 2019, with the bulk of modern OS's 128GB is really a hard sell. I don't think anyone should be selling a laptop with such a paltry amount. Apple isn't alone, as plenty of other makers do the same and its no better in windows. I'd say its probably worse given how windows likes to hold onto applied updates.
Has the bulk of OS X/macOS changed so much since the day of the 64 GB MBA? Has the bulk of commonly used apps changed significantly in that time?
That 64 GB MBA would have come loaded with Leopard. System requirement for Leopard was 9 GB disk space.
Today's 128 GB MBA/MBP come with Mojave. System requirement for that is 12.5-18.5 GB (we don't yet have a GM release size for Catalina). So yes, we need 3.5-9.5 GB of additional space today than in 2008. 38%-105% growth in the OS size sounds very substantial. However, as a percentage of 128 GB... that's an additional 3%-8% of the disk. That 3%-8% might be the straw that breaks the 128 GB camel's back, but the real elephant in the room is whatever's occupying the rest of the disk.
Between Leopard and Mojave the minimum system requirements have fluctuated. Snow Leopard dropped precipitously to 5 GB, Lion bumped up to 7 GB, Mountain Lion - Yosemite 8 GB, El Capitan and Sierra both 8.8 GB. High Sierra finally eclipsed Leopard, calling for 14.3 GB, and Mojave's variable 12.5-18 GB means it's either better or worse than High Sierra, depending (depending on what, I'm not sure).
One aspect of the OS that has grown is caching - far more substantially than the actual OS code base (buffering of media streaming and cloud upload/download/syncing, auto-save/crash recovery, tabbed browsing, search indexing...). Statistics on that are hard to come by. It's especially hard to pin down because cache sizes adjust based on available resources (at least, they do when the OS and apps are behaving properly). This factor is well hidden from users - hidden directories, no call-out in storage utilization graphs, etc., no doubt because if it was out there in the open users would try to manage things, with the vast majority mucking up their systems in the process.
Meantime, over in App Land... The "Discipline of Delivery-by-Download," "The Discipline of Mobile," and the overall maturation of feature sets seems to have kept the size of apps well within bounds. I don't have historic statistics for the size of the typical Applications folder, but my overall sense is that it hasn't changed much, in one direction or the other.
Among the components of storage (OS, Apps, and User Data), as always we have to look towards User Data as the major culprit. We add more music to our iTunes libraries, we save every photo we've ever taken, and retain every document we've ever created. We move everything from one machine to the next, and with each computer purchase we have to allow for continued expansion.
I'm not going to argue that 64 GB was an adequately-equipped computer, even in January 2008. Heck, Apple moved from 64 to 128 GB minimum Flash with the Late 2008 MBA. Since then, 128 GB Flash has held steady as the basement configuration. In terms of perception... it's been a very long time at 128 GB. And the cost of Flash, while not dropping in cost-per-gigabyte the way spinning HD storage has across the decades, certainly has dropped enough that 128 GB of 2019 Flash is far cheaper than 128 GB of 2008 Flash.
But as I argued in a previous post, the use of cloud storage can have a significant impact on local storage requirements. The ability to shift data to the cloud is one factor that has changed dramatically since 2008. Even a conservative user of cloud resources can probably manage to move 3.5-9.5 GB of their data to the cloud in order to compensate for OS growth.
While (as I stated in a previous post), 128 GB is likely inadequate if you carry your entire house on your back, I think 128 GB remains a viable configuration if the cloud is embraced. YMMV.