The guys in Maiden have been playing for 40+ years, thus I hardly think tuning down a half or even a full step would be any issue for them. Same with using a capo (if needed). At some point you have to do what’s best for your singer’s voice.
Now, if it’s Bruce who is opposing downtuning, then that’s his own fault.
Do Maiden even play in accidental keys (e.g. Ebm, C#m, etc.?) Just going off of memory it seems like most of their stuff is in Em or Dm. I think Flight of Icarus is in F#m, but that’s not an uncommon key for metal. Fallen Angel might be in F#m, too, but that’s all that I can remember...
Also, I know that Adrian has used dropped D tuning in several recordings (Ghost of the Navigator, Rainmaker, Mother of Mercy, etc.) Has he ever used full downtunings with Maiden? Like full step down, for example?