I didn't like Corrin when the trailer started, but I'll admit the chainsaw sword and dragon powers won me over. If he was just another generic fantasy swordsman, THEN I'd be upset.
(Having a waifu version also helps.)

Poeple seem to not realize just how popular Fire Emblem became globally. Awakening exploded the franchise into mainstream status, pushing it above Mother, Metroid, DK and more. It's crazy how popular the series became. But since it became popular SO quickly, people weren't gradually pushed with FE characters, they just threw them in there.

Corrin is a mix of popularity and making sure people know who he is so FE KEEPS being popular.