Buddy, it wouldn't be just a model update. It would be the full package.
New lore that would solidify their place in Runeterra, drastic design changes to be consistent with that lore, a completely new VO, a full new set of animations, new/update VFX & SFX to go along with the gameplay changes & a new model for BOTH champs. Since it would be a dual update, it means they would have to worked on at the same time.
For Kayle, she will recieve drastic changes in gameplay along with her update. Morg will likely stay the same outside of any minor balance changes.
Yeah, I don't think it's one of those things that "shouldn't take that long".