I think that having a working interpreter where you can type in assembly programs would be really useful
Actually, this decision was made due to a lack of memory on the Arduino. Assembly is far less complex than C++, so it will take less memory and be easier to implement. If you have an alternative suggestion, let me know.
Assembly is far less complex than C++
There are a few of us who only ever programmed in assembler from way back in the early days, and I know I'm happy to now use something more portable and easier to maintain and understand months or years into the future.
There are far fewer commands to implement in assembly, which means writing an interpreter for it will take far less memory and will be easier to implement. The only other language that I can imagine implementing on the Arduino is BASIC.