I don't think anybody should claim that Crosby's defense (in isolation) is anything special. In other words, his pure shutdown ability (which is how players like Bob Gainey and Guy Carbonneau contributed) is roughly average.
But, what ultimately matters is how much a player improves his team's goal differential.
Here's a post where I do a deep dive into Crosby's two-way play. I looked at 15 of the highest scoring forwards from the 2010's, and evaluated how their teams performed (at even strength) when they're on vs off the ice. A young Connor McDavid ranked first, but Crosby was second (he was immediately ahead of three centers who are widely accepted as excellent two-way players - Bergeron, Toews and Kopitar).
Overall, Pittsburgh performed vastly better when Crosby was on the ice (their ratio of goals for to goals against was about 38% higher). Having looked at this for previous decades, this is near the limit of what a single forward can do. It was the result of elite offense and adequate defense. So, the critics who claim that Crosby wasn't anything special defensively are right. But that's also irrelevant - a star player should make their team better when they're on the ice, and few players in NHL history have done that as effectively as Crosby).
(An important note - the data in that post is now five years out of date. At some point, I'll update it for the 2020's).