Federation Force was hated not so much because it was a bad game, but rather because fans felt it was a slap in the face. In 2015, all we had was rumors that somebody might be working on a new Prime. A lot of people were holding out hope that E3 would give us some information about Metroid Prime 4 and instead we were given a spinoff that nobody was asking for with no promise at all for a Prime 4, hence the backlash.
The reason why the Dread announcement is being so well-received right now is because 1) side-scrolling Metroid is not a spinoff and has legitimate historical roots in the franchise and 2) people know that a Prime 4 is being worked on and the announcement of Dread reaffirms to fans that Nintendo hasn't given up on them. That said, I do not believe that Nintendo should take this to mean that they can delve back into spinoffs without a Prime 4 being released. I would not touch Federation Force or anything like this again until Prime 4 is out and well-received by fans.
Metroid fans see their franchise as the sci-fi answer to Zelda that is on the same level in terms of quality, not a B-rated Nintendo IP like Paper Mario or Mario Tennis that gets installments of sometimes questionable quality. Zelda got BotW1 and is now getting BotW2 and so its fair for Metroid fans to want to see their franchise get at least one game that moves the series into that ballpark.