This is an issue we've actually put a lot of thought into, but it's more complicated than it seems. There are many potential problems we could run into either moving forward or backward with the FrontEnd codebase. The issues are exacerbated because the Raspberry Pi is a very modern Linux system software-wise despite having hardware that's (in some ways) more comparable to much older machines.
In order to avoid opening up corner cases, we've settled on tracking the Mathematica 10 codebase closely and using configuration to disable features (and removing some things from the layout). A lot of the improvements in prediction did come with some cost to the layout size.
We originally thought people would use the terminal interface (type "wolfram" on the command line) instead or use a text editor due to performance, but the Mathematica FrontEnd carriers a lot of appeal. So, we're working on some new developments that should make things much faster and more flexible on systems like the Raspberry Pi. If we can get there, I think we'll try to make things as friendly as possible.
Hopefully, in time, we can justify to our users why we're investing our time moving forward instead of backward on these things.
