I believe the Switch will, and should, have a mid gen refresh similar to the PS4 pro and X1X in 2020. At the very least. it would be announced no later than this.
However it will be handled more like the NEW 3DS rather than the PS4pro and X1X, where more demanding games will be "Switch 2" only and less demanding games will be released on the OG Switch and the Switch 2. The target for the Switch 2 will be 1080p 60fps minimum while docked and remain 720p 60fps on the go, allowing it to still run the rumored PS5 (I believe to also be coming 2020) multiplat games at lower res as it will surely be a 4k machine. It will feature the next chipset beyond a tegra X2 or a heavily modified version of it to achieve this.
Doing this will allow two things. It will allow the OG Switch to live longer without totally shafting the install base while allowing the Switch to a healthy existence in the PS5 generation as a hybrid console. The other thing it will do is allow the Switch brand to continue to deliver "next gen" console games on the go with concessions where necessary. I believe it is pivital that the Switch 2 can do this as this is what makes it different than, "just another handheld."
This will leave the PS5 as the default 4k console of choice, the Switch as a capable hybrid competitor while offering basic HD while in docked mode, and XBox further exploring games as a service (likely by dragging out the X1X to exist alongside the PS5, as it would be too early to release yet ANOTHER console from them).