I worked at a small company without a kernel dev and we periodically looked into the code to solve problems. I don’t know how much we upstreamed, but we relied on Linux so it was either the or try to get someone on the mailing list to care.

It’s really not that hard to look through the kernel source, it’s pretty well written and documented. It’s a lot harder to be a kernel developer writing new code, but finding bugs and contributing fixes isn’t that bad.