Realistically I think Ronaldo has one or two more seasons in Europe yes, do you honestly believe Ronaldo will still be in Europe at 37-8 years old playing as much as he does now?
Giggs made it to 39-40 but he wasn't playing as often as he used to, for a player like Ronaldo at that age he won't be moving around as much in games not as much as he used to if he wants to play a lot of games, so sacrifices will be need to made in other areas of the field.
If Ronaldo is still playing in Europe when he turns 38 I'd be very surprised, at 37 I can see him moving to the MLS.
Ibrahimovic is obviously the only player I can think of thats playing at 38, but I just can't see Ronaldo being at Juventus that long, if Juventus keep him fair play to them, but his wages at some point don't match his output and he would have to take a pay-cut.
Ronaldo is one of the greatest players to grace the game, but I can't see him being at the top top level in 2 seasons from now.
He will still convert his chances in the box, and he'll still obviously convert penalties, but what else would he offer the team at 37-38? Leadership and Experience maybe but is that worth his current wages at that age?