It seems like a neat idea, but I would have three big concerns (personally.)

1. If the modules are expensive or not interesting enough, the swappable parts becomes a problem, not a feature. If I just buy the modules I want and never change them, then why do I want interchangeable modules? This means I _have_ to want a phone that does weird things. (And if enough people want weird-thing-having phones, they become standard features, and then swappable modules looks bad again.)

2. I want the simplest phone that does everything I want. Not the most complex phone that does everything everybody else wants. Where does this phone fit?

3. Version issues. If this thing breaks backward compatibility or needs an updated core component, you might be buying a whole new phone anyway. In which case, the gimmick doesn't add value, it adds cost. The phone has to feel like an upgrade, not something that will hold you back because the module vendors can't upgrade at reasonable cost.