Is it often that Resh Lakish would argue with R Yohanan on the basis of mesorah?
Seemingly, R Lakish has his mesorah _from_ R Yohanan, so he might argue based on his knowledge of how swords are made. Of course, one might read his story as he was a TC before becoming a gangster.
Also, note that R Yohanan “seduced” RL with the offer of his sister as a wife – and RL took him on the offer. Presumably, they waited for the chasaneh until RL showed that he is serious in his learning, so his learning did not diminish his interest in RY’s sister …