I understand the appeal of minimalism but in reality most people use messaging applications. Cutting them off from their existing networks is not a viable plan in my opinion. The developers even recognise this in their FAQ, where after stating they don't support Signal or Messenger they say:

The Light Phone II can also used as a complement to your existing smartphone, for taking a break and going light, either by swapping your SIM between devices, or by getting a second phone number for the Light Phone II.

How many people will pay 800 dollars for a less functional version of a product they already own?