It's not just Messi and Ronaldo (Messi has to go first!). They have been performing at an insane level week in, week out for years.
To be world-class, you have to perform at an extremely high level, on a very consistent basis, be an inspiration to others around you, always give your best (you might get a bit of leeway on this one if your end product is spectacular), and regularly win games for your team.
Pogba doesn't deserve to be bracketed with people who do this now, such as Kane, de Bruyne, Silva, Aguero...well most of the Man City team...Salah, Mane, Lewandowski, Reus, and numerous others. You could even argue that Fellaini has actually won more points for United than Pogba, despite the fact that he obviously has about 1% of the talent.
Pogba doesn't perform at anything like world-class level. He has a world-class social media profile. He has a world-class stylist. He has a world-class agent.
What he actually produces on the pitch is absolutely nothing like world-class, let alone should he ever be compared to Messi and Ronaldo.