I think that there are two things at play here: Plugin developers who lose interest / desire / move on, and the lack of clear deliniations and steps in the wordpress development system.

The first part is easy. Developers move on. They had time when they were in school, but don't have time when they are working. Or they have time, but they are putting it towards their own design business and not making it easier for everyone else. Perhaps they got tired of the constant need to try to keep their plugin compatible with an endlessly moving code base target.

Thus brings us to point two, the lack of a solid version steps in wordpress.

Normally, the big numbers of version indicated a new code base or significant new developments. That is a clear sign that the target core has moved. Instead, Wordpress has sort of constant evolution, patching and changing and adding features, deprecating functions, and so on in a sort of random way. Making significant changes (such as the customizer, deprecating functions, and the like) should be done at a X.0 level. Making those things a .1 different (and often combining it with security patches) means that it's hard for developers to stay on target.

My belief is that Wordpress needs to:

(a) set 4.4 as the final version in the current development process. Issue ONLY security patches as need be and declare 4.4 as a stable version. Go through the plug in directory, contact everyone involved, and give them the change to bring their plug ins up to date. Making everything possible 4.4 compatible, and if not, remove it or otherwise make it available in an archive of "untested extra plugins".

(b) Begin work on version 5.0 of Wordpress. Take a big step back and consider ALL of the code and all of the functionality, and consider what is best without as much consideration for the past. Clean up code, fix issues, add a pile of new features, clean out ALL of the old, no longer supported legacy stuff, and generally build a new wordpress from the bones up. Make sure it all works, make sure you are happy with the concepts, and let the developers in on Beta to start working on 5.0 compatible plugins.

At that point, Wordpress has a legacy 4.4 version that would be security maintained for say 2 to 3 years, and a new 5.0 version with all the toys, all the bells and whistles, and none of the legacy stuff to get in the way. New themes, new plugins, new style... take a bold step.

Honestly, trying to clean up the theme and plugin libraries and backlog of submitted stuff may be a losing battle.