He will be in the top tier by the time he's done.
Based on what? Scoring lots of goals? There are many players throughout history who have scored lots of goals and aren't even a consideration in greatest of all time discussions.
Given his conditioning, he still has at least 3-4 years at the world class level, followed by another 3-4 productive years after that. By the time he's finished, although he won't have any World or Euro cups, his career goals scored will be sufficiently higher than the likes of Cruyff and Maradona, and nearly all of them will have come in big European leagues (unlike Pele's which mostly came at Santos).
His career goals
At that point the only argument keeping him out of the top tier will be lack of World Cups, but then again that may plague Messi was well.
No doubt he will score lots more goals during that time, but will he lead his teams to anything or be a key component in them doing anything of note? In the teams he's had around him since going to Madrid, all set up to supply him constantly, he's doing fantastically well in one regard, but not much in others.
Is it fair to anoint him with such lofty titles and placing in tier lists if he hasn't been as influential as anyone he's supposed to be usurping? Lots and lots of goals scored really isn't a pre-requisite, you only have to look at the names to see that, what usually goes hand-in-hand, though, is those with ridiculous GPG's also are the key component in leading their teams to the biggest trophies on the biggest stages provided to them and not falling short when they are needed the most. Ronaldo doesn't have many big games at the very highest level of pressure where you can say he was exceptional or where it could be considered a performance for the ages - especially when measured against the level of player mentioned in the bunch of names I provide below. It goes against him, badly, and always will unless he can turn it around between now and the end of his career.
The one thing that has always bugged me is the need to win a World Cup. Messi may never do it, but IMO will likely wind up as the best ever, with Ronaldo not very hard behind.
From 5 of the usually considered top 5 of all time (Pele, Maradona, Di Stefano, Beckenbauer and Cruyff), only 3 have won the world cup, and two, Di Stefano and Cruyff have not. If you expand it to a top 10 (take your pick from this selection) and include the likes of: Puskas, Platini, Eusebio, Zico, Ronaldo, Best, Charlton and Zidane, you only have two (or 3 at most) more World Cup winners, which leaves us with at least half of the established top 10 not having won the World Cup.
Delivering
great, all-time regarded performances on all the stages available to a player is far more of a factor than simply winning the world cup as a bit-part player, so I don't think a player has to win the World Cup, unless, you're talking about being the equal or better of Pele and Maradona. Then it simply must be mandatory, in my opinion, and hugely disingenuous to disregard.