Swiftkey already learns quite well. For example, I often type "Hey what's going on?" and it has now learned that if I type "Hey" it should suggest "what's" and so on, so in 4 taps I have the whole sentence.
What it doesn't have, is the context-aware switching. That is something really neat about the new keyboard. Switching context from a meeting being "postponed" to "boring" is great.
And the keyboard on Windows Phone is also really great, especially in WP8.1. So Apple needed to improve the keyboard, they were already far behind the competition.
It does. It sometimes blocks the main interface from updating (doesn't seem perfectly optimized or parallelized yet). It's also on an iPhone 4S which is the oldest iPhone they support.
What it doesn't have, is the context-aware switching. That is something really neat about the new keyboard. Switching context from a meeting being "postponed" to "boring" is great.