If you consider total cost of ownership including long-term maintenance costs, it means building has not always been cheaper than buying. I think what's changing is that it's now becoming dramatically cheaper to build AND operate AND maintain "good enough" bespoke software for a lot of use cases, at least in theory, below a certain threshold of complexity and criticality. Which seems likely to include a sizeable chunk of the existing SAAS market.

I can't believe I'm saying this, but I guess you don't even really need to maintain software if it's just a tool you hacked together in a week. You can build v2 in another week. You'll probably want to change it anyways as your users and your org evolve. It's a big question for me how you maintain quality in this model but again, if your quality standard is "good enough", we're already there.