World Tour races are harder than racing criteriums and the world Tour attracts the best athletes. But don’t assume someone like Sagan could just ride away from a top national level crit field in the US. i don’t think a national level crit gives too much away on the last hour of a grand tour sprint stage. While the guys winning top crits in the US might not be able to race 4 hours for 6 days in a row then sprint, the fast guys can sprint as fast or faster than Sagan after 90 minutes racing once or twice a week. Sagan would of course be a star but he would not win every race.
Now, if Sagan was in the typical race Norcalcycling is posting videos of, he’d do better than in a top national race but, he still would not win 100% of the time. His strengths (endurance plus a sprint) would be blunted a bit in a 60-90 minute race plus even great sprinters end up out of position now and then and get beat by slower guys,