Seems pretty accurate reason for me, especially the Jayce/Viktor being shipped before Arcane so they already have fanbase before.

Especially since Mel is an original character, which the previous Jayce/Viktor shipper who I assumed also know league, will need extra time to learn to know more about this original character for them to love this character compared to other established champions character in the show

For me it’s like, imagine they make an original character who become love interest for Vi or Cait. People who ships Cait/Vi before will naturally pissed and will still keep shipping them even if the ships not being canon in the show. They can learn to love the Vi/OC pair, but it will take time imo compare to, say, Vi/MF pair if somehow MF introduced to the show. Because at least they know MF from the League.

Just my personal opinions though