I don't think the feature as plugins model is the issue, but more of a general issue with getting things all the way there. Take the JSON Api, it's already in use by many sites using the plugin and it's a big success. But it hasn't landed in core and it doesn't look like it will in the next release either. Version 0.1 was available around June 2013. It has been teased as an inclusion in various releases since then, but it still looks like a long shot.

For some reason development just slows down. Maybe this is in part because there isn't a strong enough organisational drive to commit to certain features and simply get them done all the way by a certain time frame.