Let's look at the track record.

Backwards compatible handhelds:

• GameBoy Colour (to GB)
• Gameboy Advance (to GBC)
• Nintendo DS (to GBA)
• Nintendo 3DS (to DS)

Home consoles:

• Wii (to GC)
• Wii U (to Wii)

Not backwards compatible:

• SNES
• VirtualBoy
• Nintendo 64
• GameCube
• Switch

Lack of backwards compatibility to the previous generation has usually followed from a change in media format, and even then there has been a willingness to make an effort (the DS with its two slots being the prime example). Backwards compatibility seems to be a good way to ensure a wide selection of games at launch, and the negative aspect (not being able to sell the re-releases of the same games yet again to those desperate enough) seems to be outweighed by the positive (availability of games at launch; maintained interest in games from previous generation).

There's no real reason for Switch cartridges to grow any smaller, and I doubt they'll go back to discs. So I would say there's a pretty good chance of backward compatibility.