While I would not really say better or bigger (personally Let's GO is not super designed for my Pokemon playstyle preferences, though that is partially the fault of Kanto itself), I do think that a combination of A: The long bubbling issues in the fandom, which had been brewing since at the latest Alpha Sapphire and Omega Ruby (with the Battle Frontier's handling) if not XY, and built up over time from there to culminate in Dexit, and B: The first Pokemon game rush might lead to lower sales. 

Look I don't agree with the entirety of the build up of rage: I actually really like Sun and Moon, but Pokemon has not done the best at handling the post Dexit fallout. It almost reminds me of Fallout 76 and Bethesda, except that I can safely assume that Sword and Shield will not run like a Corgi with a hat of chocolate blinding him.

Sword and Shield will work, and not 'just work', and I do see Pokemon making money back and then some. But I can easily see a scenario in which there is a notable sales gap, perhaps 500,000 to a million, that can be taken as being the fault of 'Dexit'. Might not seem like much, but that would be noticed by the corporate heads.

Dream scenario in my end is Nintendo basically forcing Pokemon off the yearly cycle and forcing better fan-developer communication response in later games.

Truthfully, only some hardcore gamers care about the Pokédex “issue”. The real question is, how many people that care about the dex were going to buy the game in the first place? I can’t imagine that being a large number. 500k is wayyy to much. Is the super hardcore fan base, someone that would care about the dex, even that big?

Hardcore isn't the only issue: you seem to be underestimating the connection people have with Pokemon as a species. The series has spent 20 years+ telling people to value each and every Pokemon, and that isn't just the popular ones. 

From memes about top percentage Ratattas to the odd cult following Stunfisk has, from the lewd lovers of human mons to the ones who always want a Dunsparce evolution the fandom takes on many forms for many mons, often absent of competitive value. 

There is also how Game Freak has handled the issues: Game Freak has approached the issue as being related to graphics, and that has drawn out an issue in the series that usually gets sidetracked. 

For more information look up the move double kick, or 'that tree'. 

Now I am not unaware that many moves, like Razor Leaf and Earth Power, are better looking, but because Game Freak brought up graphics that in turn led to people getting on them about graphics that usually don't. 

Also I just want to remind people that 'hardcore' aren't as much a minority as one might think. I mean hardcore were probably the ones who abstained hardest from Battlefront 2/V and quite possibly Fallout 76, and look how those turned out. 

I retain the '500,000 to a million' possibility from that, and then Nintendo going to Game Freak and going 'Fix your shite', or even just going 'oops, bad call on our end, let's change these things'.

Hardcore isn't the only issue: you seem to be underestimating the connection people have with Pokemon as a species. The series has spent 20 years+ telling people to value each and every Pokemon, and that isn't just the popular ones. 

From memes about top percentage Ratattas to the odd cult following Stunfisk has, from the lewd lovers of human mons to the ones who always want a Dunsparce evolution the fandom takes on many forms for many mons, often absent of competitive value. 

There is also how Game Freak has handled the issues: Game Freak has approached the issue as being related to graphics, and that has drawn out an issue in the series that usually gets sidetracked. 

For more information look up the move double kick, or 'that tree'. 

Now I am not unaware that many moves, like Razor Leaf and Earth Power, are better looking, but because Game Freak brought up graphics that in turn led to people getting on them about graphics that usually don't. 

Also I just want to remind people that 'hardcore' aren't as much a minority as one might think. I mean hardcore were probably the ones who abstained hardest from Battlefront 2/V and quite possibly Fallout 76, and look how those turned out. 

I retain the '500,000 to a million' possibility from that, and then Nintendo going to Game Freak and going 'Fix your shite', or even just going 'oops, bad call on our end, let's change these things'.