It's more general I think: it's hard to build a SaaS without a head start. In your examples, having ~10 launching customers is such a head start.

But there's also plenty SaaSes that started building tech for something else, that didn't pan out, but they realized they could sell the tech separately. Eg close.io did this and so did my company talkjs.com. There's likely many more examples.

But I agree that if you have to start without something to sell and without customers ready to pay, you're definitely in for a rough ride.