Obviously Ronaldo would have gone down as the best player of this generation, but you can also make that argument for others, like if Beckenbauer was never born, Cruyff would have a World Cup win and would surely be rated even higher. If Maradona was never born, Platini's reputation wouldn't have fallen from being in contention for the greatest of all time to just 2nd best to someone else in his own era.
I'd argue that Messi and Ronaldo massively benefited from each other, pushed each other to higher individual peaks and allowed the media to go crazy with that rivalry in a never before seen way. It doesn't even matter who of the two is rated higher, they clearly helped each other's reputation.
And it's unlikely that Real with Ronaldo would have dominated Europe. Even though Barca pushed Real to raise their level (who knows if that had happened without Barca's superiority), Real with Ronaldo still came short against a variety of teams (they went out in the CL against Lyon, Bayern, Dortmund and Juve and only once to Barca). They finished once behind Atletico in the league now and might do again this season. Even without Messi, it's very likely that Barca would have had a sensational team around Xavi and Iniesta that would have won trophies. The new Ballon d'Or award is heavily influenced by that Messi - Ronaldo rivalry and it's not unreasonable to think that without Messi other players (Xavi or Sneijder in 2010, Iniesta in 2012, Ribery in 2013) would have won a Ballon d'Or just to balance things out and reward their achievements, like it has always been throughout the history of the award.
It's all speculation anyway, but the idea that Ronaldo would have dominated world football without Messi seems really far-fetched. Dominating football is still about leading your teams to dominance. I don't see any signs that this would have happened.