Yeah, as a general rule that's obviously sound advice, but it isn't the only factor is it?
And in the past, Liverpool have done (or tried to do) exactly that. We have a history of selling players that could easily have played on (again, not considering all the factors, like wanting to leave):
Beardsley, Case, Aldo, Rush, Souness, McMahon, Keegan, Warnock, the list is extensive
Equally, we have also kept players for far too long.
It's the most difficult part of management to get it right, and divorcing emotion from realism
Ferguon and Wenger do it best, and even they get it wrong sometimes