Being able to look at the source and troubleshoot it is not mutually exclusive to a product being proprietary (ie, source available would give you this).

There is also plenty of business I know that do everything in their power to ensure that if they go away the customer isn't left helpless (this is very popular in some parts of industrial engineering where it's common sense to hand over the blue print along with a service contract).

It's just very popular for software vendors to behave without care for the user but it's not the only business model and I think that's a common mistake of some people (ie, believing that FLOSS and proprietary is a binary property that will automatically mean certain things like you mentioned).

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.

As I mentioned, this isn't about existing vendors but potential business models (or rather, rare business models since I do know vendors that do this even with very small hobbyist users).