You can run MacOS or Windows on 256 GB almost indefinitely, even as they grow it would be five or more years before it becomes a headache, to keep 128 GB happy you have to babysit/rebuild semi-regularly.
I'm yet to see an OS which doesn't bloat as it ages, even on mobile devices. 30 GB free won't be 30 GB the next major OS update, including rollback files, logs, and so on. OS bloat is also one of the few things you cannot offload to external storage.
And I'm someone that ran off of a 128 GB SSD for almost two years.
PS - Plus increased resale value will likely pay for itself.
As a counterpoint, my 2011 MacBook Air has 128GB, and works just fine. Photoshop, coding, and lots of personal projects.
Maybe 128GB isn't enough for gaming, but I don't think that's what the "mini" is for.
Can't use it to offload and edit photos while traveling, despite the convenient SD card slot, because the disk is too small.
can't download an os update, disk full. And no, I'm not a gamer. I've been fighting that small disk for at least the past 4 years.
Obviously it depends what you do with it. A 1.5tb HDD can fill up on a week under some use cases, so it is silly to mention that yours fills up. The fact that some people can use it and it doesn't full up means it is still a viable option.
It doesn't change the fact that an OS install and updates take up a significant percentage of that 128GB, whereas they take up an utterly negligible percentage of a 1.5TB disk.
It's a shame that 256GB isn't the minimum in these new, expensive Mac minis.