If you use FOSS only, you still remain dependent on the maintainers of your stack since realistically speaking you don't have the time to understand and hack every piece of software you rely on.
Just because you yourself cannot be competent in everything, does not mean that being dependent on a monopoly is the same as having to buy certain things from people who are more competent at producing or maintaining them than you are.
Noone guarantees you that a year from now you will be able to find anyone who is willing to fix your plumbing. That does not mean you might as well have proprietary Microplumb products installed in your house that noone can repair or extend without permission from Microplumb Inc.
That's just the consequence of professional specialization. But the expected consequence with FOSS is that, if you can't or don't want to do it yourself, you can always pay a third-party specialist to go in and fix your copy of the software. In fact, it may be further expected that in this case, you'd have multiple competing specialists to choose from - just like it is in e.g. appliance / home / car repair, where you can go to whichever repair person you like and they'll fix stuff for you.
You can observe that most software is leased, not sold, thus restrictions apply. And indeed - the same holds in physical world; if you're the house/appliance/car, you might not be able to use it the way you like, or get whoever you like to repair it. But that's precisely the problem! Renting instead of owning has serious drawbacks, and Stallman is fighting against those, trying keep us at something more akin to personal ownership model.
