Ugh. Everybody seems to be reading into this way more than is actually there.

The problem is simply one of managing dependencies. WordPress does not have plugin dependencies, and trying to fit plugins into a "code library" mode is a bad fit, at best.

There are assorted problems with "libraries" in computing in general. We all know about "DLL Hell". The problem with shared code libraries is that the libraries need to remain essentially backwards and forwards compatible, forever. If code A and code B depends on library C, and library C makes a breaking change in some update, then all three pieces of code need to be updated simultaneously to avoid breakage. Updating only one of them will break something.

WordPress has not solved the code library problem, and even plugin dependencies as currently thought of won't be a general solution to it. The general method that works in the most cases is the same as that of "static compilation". Put the library in the specific code that is using it, so that no breakage occurs from incompatibilities. Each piece stands alone.

Now, in PHP, this introduces special problems. In the long run, things like namespaces and such can help here, but we're still working towards moving that PHP version bar up off 5.2, yeah? Can't leave millions of people behind.

So, do your best, is what I'm saying here. But don't try to use the plugin directory as some way of introducing shared code libraries. We don't need it at the moment, it doesn't work very well, it causes problems for users down the line, it causes confusion to users who don't understand why something takes multiple plugins, and the WordPress code doesn't even support plugin dependencies yet. See the issues?

At some future point, when the state of things changes, then the plugin directory will naturally change too. Nothing is ever permanent. If you want to help out, then by all means help out. Figure out solutions to these problems. Make a plugin dependency solution that actually solves some of these problems instead of simply creating new ones. Encourage hosts to upgrade their older PHP 5.2 customers to PHP 5.6 or PHP 7.0. Help raise that bar so that we can have nice things. Okay? Okay. :)