This whole discussion about whether the very successful sequel will get in front its the very successful prequel
More BOTW owners means more people who will pick up the its direct sequel and TOTK being a major seller means more people will want to pick up BOTW to see what all the hype around the series is about. They each boost the other up. TOTK hasn't even been through its first holiday season yet. It's gonna be featured in Nintendo's holiday ads and it will get some "free" marketing come award season.
An interesting thing about first party software in general is that more titles are becoming evergreen. Nintendo titles maintain their relevance meaning the games don't become obsolete once another title in the series releases. Sales of other Mario games aren't going to collapse because Wonder is out. A kid getting a Switch and Wonder over the holiday will go back to play titles like Mario Odyssey, 3D World, and yes, New Super Mario Bros. U. Think of all the choices a relatively new Switch owner has. They got 5 or 6 Mario platformers. 4 Zelda titles. 3 Kirby games. 5 Pokemon RPGs. People still play Smash Bros. Ultimate and there's a ton of content for a new buyer to experience. 5000 tracks in Mario Kart 8. Then there's the rest of Nintendo catalogue. Even just being a Nintendo fan in general means years of content to get through so games will keep selling well beyond their initial launch.