Some plot points of One Piece really go over people's heads. SMILE users are not meant to replace the top dogs. They are to replace the
foot soldiers. Kaido had 500 SMILE users, but what if it went to 5,000. 10,000? Imagine 10,000 semi-DF users.
Big Mom wanted the alliance with giants not because every giant is a High Tier, but the collection of thousands of giants would be devastating.
Hajrudin was one of the strongest giants and he got taken out by a single G2 punch. Are giants useless too?
When giants plan failed, she wanted Germa's iron soldiers. Kaido's SMILEs users are like that. Elite foot soldiers.