The point is that software labor is infinitely replicable and the economy assumes labor is a continual process which can be taxed. If there's no labor to do then I'd imagine the people who plan the economy would want to invent some. Fact is that technologies like the Java crusade of the 90's or the Rust crusade we're seeing today create jobs. If it weren't for them we'd all be dusting off FORTRAN programs from the 1960's which got the job done.