At this point in time ... well, yes.
Just about any software that a business needs is, at this point in time, already written, with few exceptions.
For almost any MVP, at this point in time, you can gauge market interest using something that already exists.
> I wonder how this stacks up against other software that yielded massive public utility, like UNIX, or the internet.
Those were different times, and even those different times things like UNIX were mostly differentiated only by sales-efforts of the hardware vendors.
There was a time when people had to be sold on the idea of the internet being at all useful. It had to be sold.
At this point in time ... well, yes.
Just about any software that a business needs is, at this point in time, already written, with few exceptions.
For almost any MVP, at this point in time, you can gauge market interest using something that already exists.
> I wonder how this stacks up against other software that yielded massive public utility, like UNIX, or the internet.
Those were different times, and even those different times things like UNIX were mostly differentiated only by sales-efforts of the hardware vendors.
There was a time when people had to be sold on the idea of the internet being at all useful. It had to be sold.