The Japanese Wikipedia article on Sho Shogi also describes Asakura Shogi, which is the "missing link" between Sho Shogi and modern Shogi, played with both suizou and drops. To prevent complications, a captured suizou leaves the game entirely and may not be dropped; the same goes for the king of a player with a promoted suizou.
I think the existence of Asakura Shogi casts some doubts on the idea that the removal of the suizou and the introduction of drops are inherently linked.
The Japanese Wikipedia article on Sho Shogi also describes Asakura Shogi, which is the "missing link" between Sho Shogi and modern Shogi, played with both suizou and drops. To prevent complications, a captured suizou leaves the game entirely and may not be dropped; the same goes for the king of a player with a promoted suizou.
I think the existence of Asakura Shogi casts some doubts on the idea that the removal of the suizou and the introduction of drops are inherently linked.