1)
Using OSX and ubuntu/linux installing rvm/rbenv followed by rails takes about 10 minutes - possibly slow download speed and need to compile ruby might slow it down, but still it is less than 10 lines to copypaste from rbenv/rvm installation guide...
Vagrant also takes some time to setup... in my case much much more than installing ruby. And then staring up and configuring vagrant takes some time too.
Anyways in both cases, installing vagrant or installing ruby and ruby on rails are initial setup, not something you do every day (hopefully).
2)
For people using windows, yes the setup seems to be harder and might take more time, but then you can just have a linux VM running or to use vagrant. (Or just install linux)
Or maybe help the community by improving the setup process on Windows and start helping others using Windows.
3 ) What if take the effort to fix the issue; and maybe even document it to say a blogpost to help the community.
4) Nor with maven (Java world) etc. or really any other packet manager,
this can probably be solved by rvm gemsets or just by taking the effort of sometimes cleaning up installed non wanted gems.
And often you have multiple projects and the odds are you tend to keep using same gems for most of the time... (you really don't want to 'uninstall' gem rails, should you accidentally remove it from your gemfile.
That being said, it is important to have as easy setup process as possible to make it easy for the community to grow.
-Jarmo