If anything, I feel like the fans forced The Pokémon Company's hand given the backlash surrounding Ultra Sun/Ultra Moon's reveal.
I'm not sure this holds much water. I mean if you use that argument then there's another argument which goes that announcing it so early is going to lead to absolutely sky high expectations. If people are thinking / talking about it for a full 2 and a half years before it comes out then they're gonna expect a seriously massive revamp / overhaul. Holiday 2018 might mean people may forgive it using a similar engine to Sun and Moon albeit in higher resolution and with better graphics.
As soon as you start talking about holiday 2019 people are going to be expecting a full, Breath of the Wild style open world and the backlash if it doesn't have that, after 2 and a half years, could be even worse.
I just don't think they would make the announcement based on fan backlash to USM being 3DS exclusive. They KNEW how much people wanted Pokemon on Switch. If they cared that much they would probably have tried to make USM cross platform with online cross play like Monster Hunter World has.
They're such a stubborn, set in their ways company that I don't think "fan backlash about USM" which, let's face it, was probably mainly found on ultra hardcore sites like this (I don't think there was much of a general fan backlash about it), would be enough to sway them to announce a new game SO far before it launches. Even by this industry's standards, announcing a game fully 2 and a half years before it's due to launch is very, very premature.