[benkant]

Mathematica not being Open or free is the only complaint I buy. As for it missing features, I'd wager adding them to Mathematica would be less work than building a CAS from scratch with them- in which case I'm sure you'll be missing features from the original.

[swiley]

We already have open source CASs (a handful in fact). So no one is "building a CAS from scratch".

[williamstein]

Exactly. When I started SageMath in 2004, by design it built heavily on Pari, Maxima, GAP, Singular, etc., which were all started in the 1990s (or earlier).