Agile does lend itself very well to Software as a Service, but I've delivered a lot of products within an Agile framework that indeed got finished. I've currently planned out 2 more sprints to finish the current product I'm leading. Any other work on it from there will be bug fixes if any are found on an Ops and Support Kanban. I tend to try to negotiate enhancements that were not in the initial Statement of Work as new projects. Of course, your experiences may vary depending on industry, product, and team.

As for Waterfall, it was invented in the early 1970's. I've also delivered a lot of products created through a Waterfall methodology. Personally, I prefer Agile because I find it to be a more engaging process than a Waterfall model. I have a hard time thinking which methodology delivers a better end product though. Your project may be better suited for Waterfall than Agile or vice versa.

This is from the perspective of someone that's worn Engineering, Project Manager, and Product Manager hats. I'm currently on a Product Owner / Product Manager role.