If Aguero wants to be considered on the same level as Messi and Ronaldo he's going to have to start pulling out some 45+ game seasons. No way would you limit someone of his talent to 20-25 games.
At least 20-25 of Messi and Ronaldo's games per season fall into the "walk in the park" category. When they play in the bigger games they usually take 10 min breathers before imposing themselves on the game, a luxury which they wouldnt be able to afford themselves if playing as a single striker.
I'm not saying that Aguero should be limited to 20-25 games, just that should be the limit for him playing as a single striker. He could top that up with 10-15 games playing in a less onerous role as a second striker. If we were to play him 30-35 times or more as a single striker, even if he's stays free from injury, by the end of the season he will have lost most of his sharpness. Its just not an effective way to use him.