I believe the knee story, he never really took enough time off to heal completely so its logical that it can end up as chronic.
Chasing Messi might turn him into Batistuta if he isnt careful.
But his current decine isnt just due to his knee, its a normal thing for players who rely on athleticism, once they aree around 30 the body naturally declines.
Same way Messi's dribble will decline when his acceleration goes.
ronaldo can still end up being a strong player with great positioning, basically doing what he does nowadays. Be Inzaghi, score penaldo's, pay for big teams and get credit on his name.
But it wont be the same as 2010-2013/14 for example.