"One of the many things wrong with the "package manager" 
    paradigm is that it encourages people to write software 
    which is a tangled web of dependencies. Software should 
    have few third-party dependencies, that makes maintenance 
    and porting a lot easier."

Or it means that you can share the fruits of your porting with others.

If you and I work on a project that depends on module foo, only one of us needs to port foo to platform bar once for both of us (and everyone else) to gain compatibility with platform bar with respect to our dependencies.

What you want instead is a way to have package managers support defining cross platform modules so that bar doesn't have to be forked and published as an entirely separate module.

    "The primary reason why people did not, could not upgrade to 
    Python 3 was because they had to wait for all the dependencies 
    of their apps to be ported. Many are still waiting."

Then the problem is a lack of tooling to aid with porting the code to the new platform. Creating a tool that parses code into an AST and analyzes the code to help define how it can be made compatible with the future version of a language should be a tractable problem for 1-2 developers.

At the end of the day package managers are first and foremost social software whose role is to aid meatware in achieving eventual consistency. We've only begun to scratch the surface of what is possible in this respect.