When it comes to Sagan I don't recall that many monuments where he was so much better than the field as Mathieu is. I don't think he ever had the same punch uphill nor the same TT capability to do solos as Mathieu has shown. He was also plagued by question marks about his 250k+ stamina throughout his career.
Sure there was more wheelsucking but Sagan simply wasn't the classics rider that Van der Poel is and wouldn't have rivaled him on neither a Poggio nor an Oude Kwaremont.
Sagan mostly suffered from this cause he couldn't actually distance all of his rivals. Cancellara sometimes could but was easier to mark overall due to his weaker sprint.
When it comes to Sagan specifically it's notable that Kristoff, Degenkolb, Terpstra and old man Gilbert managed to win multiple monuments during his reign.
If Van der Poel hadn't been there and WvA was the premier cobble star than perhaps he might have been marked out as Sagan was due to not being able to distance everyone well enough.