I don't think the idea is necessarily that you personally implement an extension, but that "someone" needs to implement it and the bar for "someone" to do so is lower if it can be done via extensions or libraries rather than having to be done in the core. That someone should share their work, of course, and that someone may find they have a feature or two they have to get into core, but even that's easier than sticking a whole whackload of changes into core.