That's quite subjective.

To someone with no money to spend, Mathematica isnt really an alternative to Mathics.

Is a pen not an alternative to a pencil? Different features and tradeoffs; it depends on preference and context. If anything it’s less of an alternative if there’s no differences, otherwise it would be byte-for-byte Mathematica and you’d be unable to choose between the two.

Perhaps it isn't for some things or if you can't install it. But that's really dependent on use case and user, isn't it?