Let me give some obvious comparison. Messi is a born play maker while Ronaldo is a born forward/striker. This, for me, is an obvious reason why Messi is more marketable than Ronaldo when age is concerned. People no longer rely on Messi in terms of scoring goals, but he's mostly been relied on key passes and assists for the team. This is why I see Messi still being pursued by a lot of clubs even in a retirement age bracket.