Backports could be the default

They do that already, libraries were frozen earlier.

The point of stable release is that all the software have been successfully tested together to
be working in a rather fluent way. That's why it's also risky to provide newer versions of
applications into a released stable release.

Maintaining both old and new version of some software has been done before, like with Octave,
but of course it requires a _lot_ of maintenance. There's eg. hardly enough resources to do
single OpenOffice.org version maintaining/testing at the moment, so it would definitely
require several committed volunteers to even think of eg. OOo 3.0 kind of cases provided as an
alternative version.

http://backports.org/ is there to provide some packages a bit more unofficially, ie. they do
not come with the usual kind of guarantee of quality.