If Let's Go was any indication, I don't believe it will affect the game's sales that much.

Like it or hate it, from a purely financial standpoint, this decision is sound. And not because "fanboys will eat up anything Game Freak makes"; in fact, it's probably the biggest fans who'll be most aggrieved.

Most people won't know or won't care, because having the full roster is irrelevant to an normal player or casual fan – ie 90% of the player base. The casual player plays each game as a separate entity, plays through the story and postgame then moves on, and probably doesn't know what a Living Dex is. What matters to them is the Pokemon available in the main game for their party, which has always been an incomplete list. The most they'd be affected is if their favourites don't make the cut – and given that they aren't hardcore fans, that'd be a pretty mild issue, not a no-buy deal breaker.

The big online groundswell of anger about this is deceiving, as these things always are. These are the guys whose Living Dexes, favourite individual Pokemon from Ruby, and perfect IV shinies are going to hit a wall. They dominate the online discussion because serious fans are a disproportionate majority on forums, but their impact on real sales is small. And given that these are people who REALLY love Pokemon, any attempts at a boycott will be rife with defectors who just can't resist a shiny new title.

You're left with those who are hardcore enough to care, yet mad enough to skip an otherwise neat new game in their favourite series. They won't make a dent.