It's a combination of Adventure Fatigue and Official Release Fatigue. There were 4 official books this year (instead of the usual 3), and IMO that is too many. It doesn't help that 3 of those books were released within a short time frame. It's too much too fast.
Horse crud, it's doing fine, not everything has to be number 1 in sales, it's doing well enough honestly. Some people might simply be tired of the sword coast already too, I know I'd like to explore the rest of the realms.
Sorry not buying into this bloat myth, we are no where near that point.
Still I think they should focus more on areas that have been neglected now, like Settings, fewer APs in exchange for more settings is a smart move because Setting books are fun to read even when not playing a game in a particular region. Plus maybe a few more focused player options books would be nice.
Still books don't have to sell nearly as many books as the core to be successful.
I think DMM is the coolest and most well done AP in the game and it can use TotYP to expand it further.