The more I think about it the more I believe that the main focus of Daiya is on ace pitchers in general rather than just on Eijun and/or Seidou as a team. Case in point, we know more about the rival ace pitchers (in terms of their backstories) than we do about half of the members of Seidou. Granted, we do learn about the backgrounds/histories of individual members of Seidou (e.g. Miyuki, Kuramochi, the Kominato brothers, etc.), but I think it’s quite telling that most of this additional information is presented via extra chapters rather than through the main story.
The only outlier that I can think of is Raichi, but even he begins learning how to pitch, despite already contributing so much to his team with his bat. I wouldn't be surprised at all if after Sanada's graduation it's Raichi who picks up his mantle and becomes the ace of Yakushi.
There’s also been a lot of discussion on what characteristics constitute the “perfect ace” with an emphasis, of course, on who in the series most embodies those characteristics. However, personally I don’t think we’ve really seen an example of the perfect ace in the series yet, which begs the question whether Terajima-sensei plans on turning Eijun into that ideal or whether the whole point is that there’s no such thing as a "perfect" ace, only aces of varying ability who have good points and bad points, but all strive wholeheartedly to bring their team to victory.
Basically I guess my point is so many people immediately assume that the "ace" in the title "Diamond no Ace" is referring to the ace pitcher that Eijun eventually grows into, but I think that it's just as reasonable to believe that the manga is not just a story about Eijun’s growth, but also a story about all the aces that he meets along his journey as well.
This is why complaints about the story turning into “Furuya no Ace” make so little sense to me because it’s not like the title of the series is “Sawamura no Ace” either. It’s “Diamond no Ace” and if we're going to be completely literal, then the premise of the story, based on the title, is about ace pitchers, what they do, how they operate within teams, how others treat/perceive them, and what it means to carry that number one on their backs. And seeing as Furuya is an ace pitcher, focusing on his growth and development doesn't stray away from the title of the series at all.