It happened to compilers, operating systems, and renderer. It's gradually happening to game engines.
That's why the largest software companies are really just network effect monopolies. You only use it because other people use it or it relies on some sort of proprietary format.
Many companies like Adobe, Microsoft and Google could sustain a dramatically lower headcount if they started from scratch and never went outside the confines of their network monopoly markets.
From that perspective a one person unicorn is very feasible if we think about it as building a network effect monopoly.
In fact a close example already exists -- Minecraft was very close to being a single person unicorn.
If we have a one person unicorn, I'd imagine it'd be something more like Minecraft than someones newsletter or Google!