They probably did not expect a superstar by 2026, but they probably figured a .350 OBP was good enough to bat first and help the team.
I'm not sure anyone expected Mayer to be a superstar in the last couple years. His health concerns have been a major talking point for a long time.
KC's meteoric rise to MiLB Player of the Year did get a lot of people thinking the chances he could be a star or superstar was high, but again, I'm not sure anybody felt he would become one instantly, at least with a high degree or certainty.
I think Sox management and many fans felt 2-3 would be helpful in 2026 and maybe more than just helpful going forward. Nobody ever claimed all 3 would be studs- ever- let alone this early in their careers.
The team hedged against Anthony by keeping 4 OF'ers and Yoshida/Romy as DHs.
The team hedged against Mayer (and his health) by signing IKF at $6M and trading 1 infielder (DHam) for 3 infielders (Durbin, Monasterio & Seigler)
The team did NOT count on KC for 2026, at all. No way they felt the BIG THREE was going to all be studs or even close in 2026. Yes, a year and a half ago hopes were high and very high for many fans and Sox management, but they did have back-up plans for all of them.