I played SMN in XI. I loved SMN in XI. Carbuncle and I were best friends; he followed me everywhere (except sea of course... those hpemdes were the bane of my existence...).

I plan to play SMN in XIV. But y'know what? I'm okay with it playing differently than it did in XI. It's a different game. I don't need to have Carbuncle following me everywhere.

If I had to guess, there will be a lore reason for why we can't use Carby as a Summoner. Perhaps Carbuncle refuses to lend you his power while you are imbued with the essences of the primals.