Shiki Fuuin is probably the most powerful sealing jutsu in the villiage, but who's to say Oro would have even had a chance to summon his zombies and create the need for a sealing jutsu if The Third had been younger. We'll never know if there was some technique he'd become rusty at that he might have used had he been younger to take Oro out before he managed to perform the summon (like a Genjutsu).

The stated reason the Third gave for using the Shiki Fuujin was that he couldn't destroy the zombies without dealing with the attached souls. Either Shiki Fuujin was the only way he knew to get rid of the souls, or it was just the most prudent way to do it given the situation but in either case his need to use stemmed directly from the casting of Edo Tensei, which might have been preventable by a younger Sarutobi.