Your question is exactly the right one. If there is a direct link of making social communication dramatically better with a new keypad, it may just be worth the investment. Just like in any other business, not making any investment is a big mistake. I think this is what has partly happened when the trad mobile phone paradigm is currently been taken over by touch in some parts of the user base.

“For a mobile phone company, for example, innovating extremely fluent UIs for novel social communication features makes a lot of sense, but redesigning the keypad layout probably doesn’t.”

There is, perhaps, an argument that social communication features would be better facilitated by a different keypad layout? In this situation, does it make sense to introduce a new keypad, take the risk, and invest in building the future of your user experience? Is there a limit to how much is achievable with today’s keypad designs?

As a broader discussion, I think we also need to weigh UI design decisions in the context of the overall user experience. This is something which may be influenced by multiple aspects of user behaviour and expectations generated by the other digital platforms they use.