That's an interesting aspect. I think the advances happened mostly in the areas of UI and intra-system communication. If it's about coding business logic for, say, supply chain or payroll system, the effort is roughly the same, despite bajillions invested in all the purple farts like BizTalk and new fancy functional language paradigms.
Are you sure? How big was the original Unix team? I suspect it's smaller than Microsoft's Windows team is these days.
If anything, I think you have larger software teams now than ever before.
you can build an ecommerce site or something like pinterest in hours. gem install devise - and your login system is already up and running. you can get a 3d game up fairly quickly with unity. try building an interactive mapping application in 1994, seriously. try implementing a facebook like button in 1994 - you'll be like crap, no ajax, when I want to update the like count the entire page has to update. pages like that did do that, its unacceptable today.
projects back then accomplished way less, and were much less refined than the products we expect today. I also think a lot of teams are too large, there is a huge amount of bureaucracy at some companies.
23 years ago it would have took a team many years to do what a single person can put out in a weekend. I dont think AI will be the single biggest driver though, mostly APIs getting more sophisticated. we (those that work in tech) have been lucky that the more efficiently we do something, the more demand out there is for our skills, and there was a ton of pent up demand for tech.
I dont know if its the situation I am in, but I already feel like demand for programmers is starting to drop. many engineering jobs is moving data around. just a new type of database administrator