CRMs aren't the best example of proprietary/closed-source SaaS. The CRM world is absolutely dominated by WordPress[1].
The reason open-source SaaS isn't more popular is that people don't want to worry about the technical details of the SaaS. I'm a software developer, and I'd still rather pay Trello a small amount of money rather than install and maintain my own open-source Trello. My time is much more valuable to me than the fee that Trello charges me.
The same is true of Gmail and all the other SaaS I use. I'm paying someone to maintain and operate these SaaS, not just to build the software in the first place.