Why does aptitude refrain from upgrading [UNAVAILABLE] packages?

Question

Why does aptitude refrain from upgrading in the example below? Is it solely because existing package metadata is not in cache anymore? If package is marked as [UNAVAILABLE], does it mean the only sensible action is to keep it?

system-config-printer-common will not be upgraded to version
1.4.2+20130920-0ubuntu1.2, to avoid breaking the following dependencies:

  * system-config-printer-common replaces system-config-printer [UNAVAILABLE]
  * system-config-printer-common replaces system-config-printer-gnome (<
    1.3.11+20120807-0ubuntu7) [UNAVAILABLE]

Answer 1

I think this is why apt-get is now recommended over aptitude for system upgrades. In this case, aptitude doesn't know how to proceed with the now obsolete package system-config-printer. It no longer exists but other things still depend on it. Various install guides suggest removing certain packages, force installing others, and/or forcing breakages to occur that will get cleaned up manually in further steps.

(I've not run into such a mess, personally, so I don't know what your best path forward may be. Using apt-get instead may get past this.)