My point is, it doesn't have to be black and white. If your expert is less available, or has too much of the same work, or you just want a backup, you just start training someone else to be an expert in that area too. It's not costlier than what you propose, it seems that it is always better to start with having a designated expert rather than to dilute the expertise so much than no one really is.

It does not work, at least did not worked for us. The problem is that in any single situation it is easier for expert to solve it alone then explain and when then there is sudden need to explain, expert can't do it effectively. Because he was not explaining for years.

Plus, you end up being in a limited box and have harder time to grow by learning new things - project structure keeps you in box and you can't easily expand it by taking tasks to learn something new.

Finally, expert is sort of fake expert - expert only because others are kept clueless. Not because he would had such great knowledge objectively, but because we decided this is only his area. There is no other engineer to discuss issues with our to compete with.

Nobody has said it should be except for you. As I said it will occur but you should strive to spread information and learning as much as is reasonable.