while i do not believe you have any obligation to license it under an open-source license, i think people who use it under a proprietary license are being foolish. a cas is an essential tool for day-to-day work, and becoming dependent on a proprietary software vendor for that usually ends badly
so in a sense that's a 'sage feature i'd like to see'
I feel similarly. While I myself might gamble on learning a tool like this, I'd be reluctant to recommend it to students unless I was very confident that it would always be there when they reached for it. Source-available does scratch that itch more or less, but it creates some reluctance.
Note that there are also features in Symbolica that are not in Sage, such as numerical integration, pattern matching and term streaming.