Let's face it. Divergence now is going to kill RISC OS stone. cold. dead.
You're going to get apps developers having to make a choice, or support both, as jbm rightly points out. If the API differs enough that various RO features start to differ on a user level, you're very quickly going to end up in a situation similar to a Linux kernel running varying systems on top (a la Gnome/KDE/etc.). Linux has a big enough userbase to support this. RO simply doesn't.
No matter how you look at it, this is going to cause one side or another to fall over unless they have a meaningful dialogue.