Interesting thought. Makes me wonder how much the writers and editors take the popularity contests into consideration. Let's a look a bit at it.
Toshiro didn't appear in the first Bleach poll since he basically hadn't been introduced in the series by then. Then he went to sixth, second and first place in the polls that followed. Despite his obvious popularity I don't think his role in the main story really increased that much, at least not compared to other captains that ranked lower on the polls, though he did often get very prominent fights. He did however get a movie that focused on him and a special chapters as a lead in to said movie.
Compared to that Shinso has ended up in twelth, fourteenth and tenth place in the MHA polls, with his presence being largely relegated to the background until just recently.
So, I'd say that Toshiro's popularity overshadows Shinso by quite a lot going by their respective series, and yet it didn't really impact his presence in the main series beyond what Kubo already planned I think. So, I'd guess that Shinso's appearance in this arc was planned all along rather than being popularity based.
Very true.
Horikoshi loves American super heroes and draws a lot of inspiration from them. His main advantage over the comics in Marvel and DC is indeed that he doesn't have to keep 60 years of continuity in mind or take into account what 30 other writers are doing with the characters.