Djokovic has won too much, has too much momentum, and dominated his rivals too much to now not be considered as the guy who comes through...he is massively driven.
By making the slam record his goal, making such a massively tough target his goal, it will mean he will not lose motivation. As soon as he wins this, the focus shifts to the next one, and the desire stays high...there is work to be done, the objective is not met. This is where he faltered after winning RG 2016, where the objective was to complete the career slam...it killed his hunger. But now by giving himself the biggest challenge of them all, he is ensuring that he never loses that motivation.