Upgradeability and ease of repair.

I can imagine a BlackBerry-style keyboard showing up at some point, or maybe even something that isn't a keyboard (ex: a mini arcade joystick + buttons for games)?

W.r.t. repairs I don't think physical keypads have been the main source of difficult or costly repairs. You can easily make a keypad repairable or replaceable without this gimmicky swappable keypad.

W.r.t. other style keypads, that's a big "if" they exist. Oftentimes unless there's a huge popularity of a product, those pie-in-the-sky future additions never get made or see the light of day. Hope they do for the people that want it, but I wouldn't hold my breath for how niche this product is.

They have an iPod wheel/button for it in their site.

It'd probably be useless for phone control, but I'd love a flac player with old iPod controls.