There is still competition to consider though. If a SaaS product has competitors that are more reliable, or more feature rich, they should win & retain more customers over time.

Data lock-in, due to proprietary file formats, makes switching software extremely difficult, especially for companies that interchange data with outside entities.

For instance, companies that do design and/or A/V work have no real alternatives to paying Adobe monthly--regardless of how little work Adobe puts into Creative Suite--because it's an industry standard and there are no alternatives that offer friction-free interoperability.

They still have to attract new customers, and investors really want to see growth not just a stable number of subscribers.