"If everyone in the next generation of game developers decides to use a pre-fab engine, who will be left to make the new engines?"
Implementing low-level systems yourself is a valuable exercise for games as much as for any other software. You gain new appreciation for the things you took for granted, and pre-made abstractions become even more intuitive because you feel more in control of what's (probably) going on under the hood. One of my pet projects is implementing graphics primitives in software - you'd be surprised at the amount of tuning and iterations needed to make them fast, clear, and correct.
Implementing low-level systems yourself is a valuable exercise for games as much as for any other software. You gain new appreciation for the things you took for granted, and pre-made abstractions become even more intuitive because you feel more in control of what's (probably) going on under the hood. One of my pet projects is implementing graphics primitives in software - you'd be surprised at the amount of tuning and iterations needed to make them fast, clear, and correct.