Most customers won't sue you. They'll terminate their business with you and go elsewhere.
You have to decide, as a business, if trustworthy vendors are available and select the right one. You can also decide that it's important enough to your value proposition that you want to "own" it.
The real reason companies hire vendors is that you don't need some mythical "FTE" that can do all the things. For the "price of a single FTE", a vendor can provide you access to a range of FTEs that are knowledgeable and capable in different domains, and you can flex between them. Thinking about RedHat, as an example, if you pay RedHat the equivalent of a single FTE, let's say $300k/yr, is that just one FTE working on maintaining Apache? A kernel contributor? GCC?
This is about fractionalization and labor specialization, not some direct 1:1 tradeoff between hiring some mythical person with commit bits in hundreds or thousands of OSS trees.