Personally, I think people are underestimating Switch's holiday season. Sure, I don't think it will be above 8M, but I do believe that 7.25-7.5M is highly likely.
Here is why I feel this way: Mario Wonder had the highest initial sales for a Mario game, but the user base for the Switch is extremely high right now and software sales are still very high too...yet Mario Wonder really wasn't as performant it seems like it should have been based on this information. Marios popularity hasn't declined given the fact that the Mario movie is the highest grossing movie based on a videogame and, I believe, the second highest grossing animated film ever made. In addition, other Mario games have sold significantly more due to their evergreen status. But having an evergreen trajectory is a lot more difficult at the end of a consoles life as sales slow down. Which leads me to one conclusion, there are a lot of people waiting to purchase the game with the best bet being parents waiting to purchase it for the family or their children with a new Switch console.
Personally, I think we are going to see continued strong sales this holiday period with a sharper decline starting next year, particularly Q4 or Q1 as we hear continued rumbling and even possible announcements related to Switch 2