So basically, the idea is that due to its reliance on user-created levels, it will not appeal to the primary 2D Mario audience who want a full-game's worth of professional levels ready-made for them? Won't the user-creation aspect draw in an additional audience of its own though, spreading through the showing off of custom levels on Youtube and such in much the same way as Minecraft, where kids will see some crazy user-created level and want to try it for themselves?
The audience for usergenerated content is small. The main reason for Minecraft's success is not usergenerated content; it's the adventure in a randomly generated world where the player can do what they want; it's the same basic idea (do what you want) that makes Breath of the Wild sell so well. What users create in Minecraft are not levels, but buildings and other things with the available resources.
8m lifetime would already be a great result for Super Mario Maker 2, but 10m seems so unrealistic considering how games that were about usergenerated content have fared in the past.