I think there are a few factors in this from what I’ve seen. Honda seem to have the best overall bike, but take nothing away from Marcquez, he is clearly more than able to extract all of its performance.
Dovi on the Ducati is a good challenge but ultimately falls short over a season.
Having watched that Amazon Prime film, I get the impression Jorge Lorenzo could be Marcquez’s biggest rival, should he get the bike he deserves. It seems there’s been some sort of shift in bike technology, not sure if it’s through rule change or just manufacturers making progress, but it seems to centre around ‘electronics’ and Yamaha has struggled to sort theirs out in comparison to Honda and Ducati.
If they were able to deliver a better bike for the riders then Lorenzo could be up there with Marcquez. He is more than a challenge for Rossi and has won the Championship himself at least once whilst Marcquez was around and with Rossi on the same bike.