This is true when SaaS is a simple widget for everyone. The problem is that when SaaS becomes a hydra designed to do a million things for a million people, the extra eyeballs aren't helping you, they're creating more error surface.

On top of that, SaaS takes your power away. A bug could be quite small, but if a vendor doesn't bother to fix it, it can still ruin your life for a long time. I've seen small bugs get sandbagged by vendors for months. If you have the source code you can fix problems like these in a day or two, rather than waiting for some nebulous backlog to work down.

My experience with SaaS is that products start out fine, when the people building them are hungry and responsive and the products are slim and well priced. Then they get bloated trying to grow market share, they lose focus and the builders become unresponsive, while increasing prices.

At this point you wish you had just used open source, but now it's even harder to switch because you have to jump through a byzantine data exfiltration process.