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Seems like a worthy successor to Sympy, although the license system might prevent it from reaching the same level of adoption.

No, being commercial prevents that. SymEngine is much more like that, and shares developers with SymPy: https://github.com/symengine/symengine, https://symengine.org/.

As someone who has not used Sympy: Why does it need a successor?

Development is pretty much dead other than some bug fixes. It's been long overdue for a major rewrite.