I dunno if releasing Warriors in January would change things much honestly. Sure there's some overlap with people who buy every 1st party Nintendo game, but I feel like people interested in FE: Warriors weren't going to be dissuaded by Mario. On top of that, there's no rule that you can only buy a game at release...the game isn't going anywhere, and will be getting DLC throughout the year that seems to be a good value so far.
Honestly, there's probably more overlap between the Xenoblade and Fire Emblem fanbase than Fire Emblem and Mario, which would have released a month before this hypothetical release date.
Completely agreed about the Echoes getting a dual release though. It's easily the best Fire Emblem on 3DS, and it's unfortunate how limited its sales were. I suppose there might be reasons it couldn't be ported easily, but I still would have preferred this game on the Switch.
NNID: Zephyr25 / PSN: Zephyr--25 / Switch: SW-4450-3680-7334