I don't see any argument that he isn't a 1C (unless you only consider the top 10-15 centers in the league a 1C).
He hasn't been outside the top 35 in points among players listed as center on NHL.com since the lockout shortened season (when he only played 29 games and was on pace to finish in the top 30). His lowest finish in that span was last year at 33rd and every other year he was among the best 30 centers in the league in actual production. He was top 20 in two of those five years. His absolute floor is a 55 point player. He has hit 60+ in 3 of his last 5 seasons and had a 4th season where he was on pace for 60 points but missed 10 games and finished with 55 points. Barring this season, he has been a 55-65 point player. From a pure production standpoint, he is a mid-low end 1C without factoring in his pace this year.
He is 14th among centers in overall points since the start of the 2013/14 season with 318 points in 404 games. Toews is right ahead of him with 320 in the same number of games played. Johansen is 12th with 327 points in 425 GP and Bergeron is 11th at 327 in 403 GP. Two of those guys are future HOF centers and the other is a guy touted as the 1C Army missed out on.
However, it is his 2-way play that makes him unquestionably a 1C. He is the best faceoff man in the league, he is fantastic on the penalty kill, his relative possession numbers are very good and he can hit those offensive totals while playing a pretty defensive role (in the 4 years leading up to this one, his D zone start rate was consistently between 53 and 57%). His lowest finish in Selke voting in the last 5 years is 33rd, but he was top 12 in every other year.
I know that I've harped on the definition of '1C' or 'top 6' a lot on this forum, but I think it is important. We can debate whether or not he is a franchise player or an elite center, but he is objectively a 1C even if his current play is unsustainable and he regresses to what he has been for the last 5 years. Personally, I think he hits 70-75 points this year playing with Tarasenko and I don't see his production falling off a cliff in the seasons to come. I truly believe that ROR will be a top 15 NHL center over his first 3 years as a Blue.