That's still after a heavy drought, though. As you said, Dread came out a couple of years ago. As you know, neglected franchises like Metroid, F-Zero, or Star Fox don't have the staying power that franchises like Mario, Zelda, or even Kirby do. It's why even though it wasn't really a new game, F-Zero 99 being announced was a big deal because Nintendo actually acknowledged the franchise. That's applicable here too with Prime 4 not just because it's finally the sequel to Prime 3, but also considering how long ago this game was even announced.
It'd be like if we actually and finally got Beyond Good and Evil 2 after it sat on the shelf for so long. Metroid may be experiencing a sort of renaissance, but that's mostly due to Dread and, before that, Samus Returns. Metroid games never came out with the frequency of Mario or Zelda games, so yeah, and with how much Prime 4 has just sat in development, it's not only a big deal, but it's still fair game to call it a neglected franchise. Or at the very least, overlooked because, again, it's never been on the level of other franchises, despite how beloved the Prime series is. The older games being added to NSO are nice, but like F-Zero X coming to Switch, it feels more like Nintendo throwing those franchises a bone rather than coming out with something new. Even Prime Remastered, while great, was nothing new for the franchise.