Having been involved in scheduled performance since the early 70s I have to say you've hit the nail on the head .. at least for some cases.
In days gone by we cut our teeth on the slide rule and AFM performance charts - all done mandraulically. As a consequence the average performance engineer got to a point where he/she had a pretty good feel for an aircraft's capabilities.
More recently, the manual things have gone the way of the dodo and, while the engineers are no less potentially competent or capable, they haven't had the opportunity to have the exposure to the nitty gritty work.
Much the same can be said for old pharte pilots versus the current crop coming up. The former had a feel for things, having had to do stuff from paperwork. The latter have been treated unfairly by only getting to push buttons and, as we all know (I hope) GIGO often rules the day.