Depends on what you consider a successor. I think we could have a “Switch Pro” utilizing a more advanced Tegra processor as early as 2020. As for an outright Switch 2/Switch successor, that operates on way more advanced tech, changes the form factor in a way that hurts joycon compatibility, etc, it could be as late as 2025.
Personally, I’d consider the Pro a successor, so I’ll vote for 2021.