They could but I bet a lot of people wouldn't be happy about that.
Not sure who mentioned it but someone gave a vague reference to the playtest survey about the release schedule and people asking for a specific amount of release. This never happened because they survey never gave a specific number. You saying you want less can mean many different things. If people were satisfied with the releases then DMsGuild and UA wouldn't really exist because people would be happy with the amount Wizards gives them.
I'm afraid Jester's comment isn't valid because 1 book a year is an extreme, just on the other side of the scale. Moderation usually runs in the middle of the spectrum.