I think CarJax was mainly about Carly having this brand new post Sonny life with a new man more than anything else. The fact that it was Jax who had a prior history with Sonny was an added bonus that they could mine but if LW had not sparkled with IR and had instead sparked with someone else ( Lorenzo and Nikolas were also potenital love interests or Carly could have been the main spoiler for Patrick/Robin) then Jax would have been left right out in the cold and probably written out years earlier than he eventually was (in 2011).
Lol I still love Sami bringing it up. The entire town is like it's been decades and we are cool with that, but not our Sami She will just bring it up casually on the reg. I wish she'd sell Belle on the black market again