Technical innovation increases the productivity of labor in industry. If production shouldn't be wasteful, there needs to be a reduction in workers. Where do all these workers go if there's no unemployment? In a capitalist economy the workers are left to die on the streets but in a socialist economy they need to get a job again, right? Does the state take the burden of cost of reeducating these workers to enter another field and factor this into calculations when introducing new technology?
I think we should also remember that what we see as labour under capitalist labour would not necessarily be the same as what we see as labour under socialist society. In capitalist society, we only tend to view labour as, to put it bluntly, jobs or activities that generate some sort of profit. If your labour does not generate a profit but is useful to society, i.e. lots of healthcare workers, the railroads, education etc., it runs the risk of being cut. And then we haven't even talked about labour that isn't seen as labour, like household activities or raising children.
When we switch to a socialist system we can also start viewing labour as something else than something that generates money one way or another. Many forms of work will rise with the implementation of socialism.
This isn't necessarily an answer to your question, but I think it's something to consider.