There have been many tangible arguments. People have brought up the following tangible skills:
Effectiveness around the net and behind the net
Backhand shot
Battling along the boards
Breaking up plays in the defensive zone
There have also been attempts to display how dominant Crosby was, that doesn't get seen in raw point totals due to him being injured in his peak, and playing in a dead puck era 2.0. For instance:
Crosby leading the league in PPG just as many seasons as McDavid
Crosby having dominant PPG and end of season point total finishes over 2nd place
Crosby showing his versatility by adding that around the net garbage goal ability to his game, snagging a 2nd Richard in the process pre-concussions
There are tangible rule changes that can be pointed to that reduced the number of head shots and leading with the elbow, blindside hits, etc after Crosby got blindsided for another concussion. He helped paved the way for a league that protects its star players more.
There have been far more nuanced arguments than the above, but there are tangible reasons that stats clearly showcase Crosby being on the same tier as McDavid. Its easy for Crosby haters and McDavid fanboys to say, "Well why didn't Crosby win more Harts?" and then when we bring up a season like 2013 where he dominated with 1.61 PPG and was lapping the league before being sidelined again by a cheap shot its like "Well he didn't win it, so your stats are useless, only trophies matter" and so why not chirp back with Crosby's Cups, Olympic Gold, and WJC Gold and showcase how he wins everywhere he goes on every level, and is the leader on the team when doing so?
"There is just nothing tangible, I just don't see it"
Well then you aren't trying to see it, in fact you are making a concerted effort to try not to see it.