You are wrong. Zoro is far from a glass cannon. If he was he wouldn't have survived hakai, a combined attack from 2 Yonko designed to beat 5 SN. Zoro wouldn't have bested Kaido with 20 to 30 broken bones. Zoro wouldn't have remained woke after Kaido hit him with TB.
Just about everyone find a style stays the same. They just get better at it. Zoro improves his cutting power. Sanji improve his kicking strength and power. Luffy improves the power behind his punches and that usually involves him making bigger fist to punch his opponents with.
It is still taking multiple people fighting Kaido to take him to this point and that's the point.
The manga said it's fate Zoro is there just like it talked about Luffy's fate.
The official One Piece magazine asked if Zoro would recreate Ryuma's legendary dragon slaying feat.