Can't fully agree on that. Boonen and Cancellara just like Van Aert now had to face best classics riders of the world at that moment. Van Petegem, Hincapie, Flecha, Ballan, Pozzato, Hushovd, Chavanel, Gilbert, Van Avermaet, Terpstra, Kristoff, Sagan.., to name a few. Every coin has two sides, maybe those riders would be bigger champions but because of those two they wasn't.Results are out of reach, but not because he is not in the same league. Remember how a young Vanmarcke was one of the only competitors Boonen and Cancellara had in the classics? Think about that for a moment. It says a lot about the quality of opposition both had/have to deal with.
To conclude:
Results aren't out of reach, but that will be very hard to accomplish,
and he is not in the same league as Cance, at least yet...