Why would Nintendo make a new version of the Switch? Because Nintendo is in the business of making money, and a new version of the Switch would likely cost less to make while being sold for the same $300 (or more!) than the current Switch costs to buy.
It's why Nintendo released so many versions of its Nintendo DS and 3DS consoles, and why Microsoft and Sony do the same thing with their Xbox and PlayStation consoles.
At the same time, new hardware re-ignites excitement — though, admittedly, the Nintendo Switch already has plenty of momentum.
It's also an opportunity to smooth out the jagged edges.
The Switch could use a stronger battery, and a better screen. The Joy-Con wireless gamepads could have a more secure connection, or simply be permanently attached to the console. It could have better support for new WiFi standards, and come with more internal storage.
In short, there are plenty of changes Nintendo could make to the Switch to make it a better console.