You're right that MM has outperformed him, but he started as a 20 year old against a 34 year old Rossi.This is the 10th year n Rossi’career where riders have been on equal rubber and Rossi’s numbers n that timeframe are pretty unextraordinary and 6 of his titles came with a tire advantage. Actually all 7 did because 09 he had a huge data advantage on the Bridgestones. Once that last advantage dried up he has 12 wins and 0 titles in 9 1/2 seasons which actually puts him in comparison to Dovi, not Marc.
Flip the script (a 20 year old Rossi against a 34 year old MM), and we'd have very different numbers.
He's outperformed Dani and been neck to neck with Lorenzo in the post-SNS era (again, being many years older than both of them).
Marc is an extraordinary talent, but it's too early to make the case for GOAT. He may very well achieve this.