I agree with Pixela. All but requiring a daily sortie grind is bad for the game.
I'm one of the many ffxi players who is a perfectionist. If I'm not working on getting BiS gear, the game is not worth playing for me. After playing for 20 years, I quit again in May this year, because spending 1.5 hours daily grinding sortie was just not practical for me. I have a job and a family. I could do this twice per week, but not daily.
Giving sublinear and almost linear reward scaling for multi-day cooldowns would have no downside and bring back players like myself. Make it factor 2 reward scaling after 3 days. Those who do Sortie daily then still have the advantage, but twice per week players can still make reasonably quick progress towards a prime weapon. Everyone is happy, and FFXI has more subscribers.