What are you on about? What multiplatform games a platform plays doesn't determine if a platform is in a gen or not.
What does then? The Switch is considered Gen 9 because The WIi U was a massive bust.
If they had back to back bust would the Switch have been Gen 10 then?
Why is the X1X not considered a new generation? The PS4 pro was barely an update but consider the original X1 and the X1X - huge difference.
What causes a generation change?
"What does then?" and "What causes a generation change?
When a company launches a successor that is on the market for at least 4 years, then that system is in a new generation. The first company to do this starts the next generation. That's the way it has always been.
"The Switch is considered Gen 9 because The WIi U was a massive bust."
The Switch is considered Gen 9, because it is the successor to both the Wii U and 3DS. Both were Generation 8 systems. The Gamecube was a massive bust and the Wii was still next gen. The Saturn was an even more massive bust and the Dreamcast was considered next gen. A failed system still counts as long as it is supported for a whole generation. That is the way it has always been.
"If they had back to back bust would the Switch have been Gen 10 then?"
The N64 and Gamecube were both failed systems, and they both counted for a whole generation. The Wii was still a Generation 7 system.
"Why is the X1X not considered a new generation?"
Because it has the same game library that the normal XBox1 does. It's just an upgrade like the DSi or New 3DS, and these have never caused a new generation.