I'm not sure why this is even a discussion or considered "news". There's always only been one Smash game per console. Whatever console comes out after the Switch is still a long ways away, so no duh they aren't working/planning for the next Smash game yet, which will undoubtedly exist. The DLC is still coming out for the current game and you literally highlighted the fact that Sakurai will only work on one thing at time.
I bet they won't make a new smash game until the next console. Look how long Siege has been around? They keep adding more DLC characters and it's doing great. I could see Smash doing the same thing.