@Milan Petrovic - Why is that not a solution? We are now cutting out themes and plugins that are older than 2 years from the WordPress.org directories automatically. Most of the feedback we've gotten on the idea has been along the lines of "2 years seems like it's too long".

People not upgrading their code is a problem to be solved, not a data point that you should be considering in order to adjust your development practices. If somebody is running older versions, then they are doing-it-wrong and you need to encourage or help them to fix this issue.