Costs don't disappear just because it's the private sector that has them. On the national economic level, waste is waste.

But I agree that with UBI people who have actually important jobs might also quit, or demand higher salaries.

Some would say that sounds fair and sensible compared to the current status quo.

About software development specifically: Note how many of the most valuable software projects have grown not from profit driven corporations, but from open source maintainers working in their spare time.