Our projects are closed source due to our clients owning the code, but I can offer anecdote. We have a client whose business operates on 2-3 very niche SaaS applications in the veterinary/animal medicine space. In a span of about 6 months, we completely ripped out 2 of those 3 and are working on replacing the 3rd one right now. We've done this with a single senior engineer working with the client between 20-40 hours per week with no major regressions. The business has been able to continue working as usual with no disruptions throughout this process.

Obviously it's hard to measure this objectively, but I can't imagine having done this pre-AI with zero downtime and having replaced those SaaS applications in that timeframe.