That's an interesting point. Think of someone getting into this stuff early in his career. How long will it take to get proficient? In my opinion, it's necessary to know the underlying principles to use the abstractions well. So 10 to 15 years sounds about reasonable.
In your case, the landscape was quite different 15 years ago. Simpler, in a lot of ways. Now think 15 years into the future and imagine how the Unreal Engine could look like then. If the changelist increases in this form for the next 15 years, how complex will it be to operate that thing? Will someone new to this still be able to learn everything there is to learn? If not, how will this affect the industry?
I disagree about needing to know the underlying principles. All you need is a sufficiently good functional understanding for the task at hand. You don't need to know about lie groups and linear algebra to rotate an object with your hand. If the engine gives you the tools, you can just use them (again, if you have a sufficiently good functional model in your head). Of course, maintaining the critical parts of the engine is another thing entirely, but even then they're divided so that each person only needs to understand a region of the code.
this is why very very few people will know all parts of the engine and dev teams would rather assign specific parts of an engine to different devs/specialists. this even make sense for a company to do that. i can't imagine what would happen if the few guys who know all were to leave the company with all the knowledge.
I think understanding how things are implemented intimately is something that certainly helps, but isn't 100% necessary in many cases until you want to start optimizing, or you are trying to do things the engine isn't designed to handle natively.
In your case, the landscape was quite different 15 years ago. Simpler, in a lot of ways. Now think 15 years into the future and imagine how the Unreal Engine could look like then. If the changelist increases in this form for the next 15 years, how complex will it be to operate that thing? Will someone new to this still be able to learn everything there is to learn? If not, how will this affect the industry?