He's definitely a curious kind of character, his arc is really intriguing and I really enjoy it! I overall enjoy him as a character. And since the manga/anime is shounen, he's definitely the most viable option as a protagonist amongst the teenagers in Shiki. That Ryuuzaki-sensei decided to turn Shiki, a mature/adult book, into a shounen manga is perhaps because shounen genre is more of his forte and it sells more. (Shiki becomes even more action-packed, while the books are more about introspections, human interactions and morals) Toshio and Seishin are definitely too old to be protagonists there.

Seishin is the books' protagonist, but even he's a rare kind of protagonist, I think. Perhaps if Toshio is made a protagonist, people will like it more. Not everyone is able to relate to Seishin. Shiki is, I think, a story about Seishin's self-discovery and change, fabricated by a story about a dying village invaded by supernatural entities. Shiki contains little arcs about other people, as participants in the whole ordeal, but it's mainly centered around Seishin. Perhaps that's why Toshio is also present a lot, because he's one of the biggest influence to Seishin's life, as a person who matters the most. The story eventually leads to their clashing and separation, because it's one of the keypoints to Seishin's change and arc. Toshio is the deuteragonist, I think.