I think SymPy already flourishes in its space: it's a great tool for users who need some computer algebra functionality with the ability to glue that together w

I think SymPy already flourishes in its space: it's a great tool for users who need some computer algebra functionality with the ability to glue that together with other code (thanks to Python). It's fine as long as you don't need high efficiency or specialized functionality. Mathematica has no serious competition as a general-purpose symbolic system. I guess Maple is the closest runner-up, but it's more narrowly focused on computer algebra. Most people only need a fraction of Mathematica to get their work done, though.