Again, 256GB SSDs in notebooks are more geared for businesses where there are either on-premises file servers and/or sanctioned/provisioned cloud storage subscriptions where most data is supposed to live long term. It's not great for personal use nor for someone who isn't loading a bunch of apps or files onto their computer.
Even with a unified memory architecture, I can guarantee you that there will be native app use cases on Apple Silicon Macs where 8GB, let alone 16GB of RAM will not be enough.
Again, I don't know that only having 256GB is justifiable by virtue of the fact that those services exist, but rather that (a) businesses typically want as little company data stored on the endpoint as possible, thereby making 256GB adequate for standard business laptop SSD sizes seeing as they only need to house non-intensive applications and the OS and (b) select home users either don't have a lot of data or have data that's primarily in cloud services (e.g. iCloud Photos, Google Drive, etc.). I would never recommend a home/personal user spec out a computer, let alone one where the internal storage is neither upgradable nor replaceable, with only a 256GB drive. But for a standard business use laptop, I would see no problem with doing so.
Apple's pricing on SSDs has always been ridiculous. That being said, if one's needs don't entail content creation, scientific computation and/or engineering, or gaming, and ESPECIALLY if one is using either external storage, cloud storage, or local server storage (as, again, is the most common scenario among standard business user use cases), 256GB is perfectly adequate. As you've said, anyone with prosumer needs would definitely want more than 256GB of internal storage anyway. But yes, in the "standard business user laptop" setting, the 2-port 13" MacBook Pro like the 2011-2017 MacBook Air that preceded it, has always been the sweet spot for that kind of user. The only difference now is that the M1 MacBook Air can also suffice (whereas the 2018-2020 Intel Airs were pitiful, even for those use cases).