As my manager said to a young me when I offered to replace our CMS, and promised I could do a good job at it, "you could probably assemble our office furniture too, but I don't want to pay you to do that either"

We have replaced many SaaS with inhouse solutions, but most of these where lacking in quality and where part of our existing core business model which we where not "owning" prior. We can flip the argument where we have lost customers and revenue due to SaaS not delivering

The gains is generally more seen outside of monetary as these SaaS solutions where holding us back for achieving our goals and improving our services to our customers. As in the end of the day our customers do not care if "insert SaaS" is having issues, it will always be our problem to own.