So what can they do to make Metroid bigger?
I think they need a good marketing hook to convince non-Metroid fans to try out the game, at least. Whether its a demo, a Metroid Prime 4 focused-direct, or something. It worked with Xenoblade 2 (though it didn't have a demo) as it was consistently present throughout 2017 as a big game for Nintendo Switch, even though the series has been historically niche. It also didn't hurt that the launch was not too far away from the initial reveal so the iron was hot enough to help Xenoblade 2 become the best-selling Xeno game in the franchise. Same with Fire Emblem Awakening and Fates. Part of the reasons why they did well was because Nintendo promoted casual mode (which the Japan-only remake of New Mystery of the Emblem had as well, but...) and provided a demo for Awakening to entice newcomers.