I do agree with you that there's a skill tree for any practical work to be done. And nodes can be simple or hard. But even if there are dependencies between them, the nodes are clearly separated from each other and some are shared between some skill sets.

If you take the skill tree you need to be a kernel contributor, it does not take much to jump over to database systems development, or writing GUI. You may argue that the barrier entry for web dev is lower, but that's because of all the foundational work that has been done to add guardrails. In kernel work, they are too expensive so there's no hand holding there. But in webdev, often enough, you'll have to go past the secure boundary of those guardrails and the same skill node like advanced data structures and concurrency will be helpful there.

Kernel dev is not some mythical land full of dragons. A lot of the required knowledge can be learned while working in another domain (or if you're curious enough).