Sure, if you're a big enough customer, you can have access to (relevant parts of) source code, and various forms of preferential treatment. I've seen his firsthand. This is the norm with big vendors that are on the market for many decades, and not going anywhere (e.g. Microsoft).

If you are a smaller fish, you have fewer of these options, because they just cost money.

Of course it's often cost-effective to quickly start using a piece of commercial software or SaaS, e.g. Slack or Gmail, as opposed to hosting your own chat or email with opensource solutions. As with any software where you put your important data, you need to know your way out, and a way to transfer these data to different software / SaaS. With email, it's reasonably easy; don't know about Slack. With open-source solutions it's usually reasonably easy, too, and it's easier to find, or at least to produce, a migration tool.

In some cases, there's just no free alternative; e.g. if you are writing a PC game, you can't realistically stay away from Windows.