The gameplay is relatively in line with the previous games and the change in linearity and narrative was an expansion on the direction of earlier titles (As you stated, Fusion in particular).
It wasn't a paint by numbers follow up, but it built on previous titles direction. They didn't reinvent the wheel they just pushed it further down the road.
The game came out in either a Monday or Tuesday and was in bargin bins by the end of the week. I don't think word of mouth travels that quickly and if the game had been something people really wanted the front loaded sales would have been higher.
Prime 4 probably makes sense but thier seem to have the staff for it busy on
other projects. A main line game doesn't. Considering what the series has been through I can see why they are doing a spin off rather than a mainline game (No indication the market will support it) or a Prime (Do we really want someone else to do it rather than Retro).
Regardless, it's just a spin off, I can understand not being interested but the salt level seems unjustified.