I know at least 5 people - a large percentage of my close network ;) - who would be interested in this device. I'm sure there's a big enough market for it if the execution is right. Though I'm personally waiting for Librem for the security/privacy aspects.
you can treat your smart phone as old boring phone by using it like a old boring phone.
Not saying that's a large market, but I do think it exists. At the 350$ price point though it seems more like it would be for wealthier people looking to signal that they're part of a sort of "offline" movement.
The organic foods industry has these sorts of seemingly niche products which are more expensive but become quite trendy because of what they represent for the customer and their peers. Not sure how well it applies here though.