Not really convinced by the first graph (and following too). According to it on a 10 year old project developers only manage to spend 10% of their available tim

Not really convinced by the first graph (and following too). According to it on a 10 year old project developers only manage to spend 10% of their available time adding features because rest is consumed by maintenance? By 10 year landmark I would expect most of software to be mature, with less new features needed and most known bugs fixed.