Messi is considered the GOAT now, by many people. What makes you think this will change as time goes by? He can only add to his legacy as he wins more with Barcelona.
You also said that you can see Cristiano winning 8 or 9 leagues by the time hes finished, and that would be as great as any other GOAT candidate. Well, Messi is already on 10, he could end up with 12/ 13 league titles. Which GOAT candidate would come close to that? Hell, which GOAT candidate comes close to his current 10?
I don't think 8-9 vs 12-13 league titles (or 10-11 won as key players) would makes any difference in GOAT standing. It only means both players are very very accomplished/successful at domestic league level. Let's just say in terms of their career, both will get a 10/10 at domestic league level. but if we look further, Ronaldo will have the edge.
Domestic league level
Messi 10/10 - Won 10 La Liga (8 as key player), scored 421 goals in the league
Ronaldo 9.5/10 - won 6 league titles in England, Spain and Italy, scored 423 goals in the league (would get 10/10 if he wins a few more)
CL level
Ronaldo 10/10 - won 5 CL, scored 127 goals
Messi 9.5/10 - won 4 CL (3 as key player), scored 113 goals
International level
Ronaldo 9/10 - won Euro, scored 95 international goals
Messi 8/10 - 0 trophy, scored 68 international goals
Individual level
Ronaldo 10/10 - won 5 Ballon D'or, scored 701 career goals
Messi 10/10 - won 5 Ballon D'or, scored 674 career goals
Fans recognition (eye test) (subjective)
Ronaldo 9/10 - most determined, most clutch
Messi 10/10 - most talented, most god-gifted