I'm a union member and a software developer. *Lots* of developers (and engineers in pretty much every engineering discipline) in my country are.
Unions are a defense against all sorts of labour abuse and every worker should be in one, but if full-on replacement actually becomes possible, I don't get why people think the union can protect them. The union can threaten to withhold labour, but what good is that if that labour has just been rendered valueless?