>That said, like C#, it is strongly associated with Apple as a “private” language (despite being open-source).

You can't use Swift on Windows. You can target practically target most operating systems with C#. C# is not only open source, but an international standard. Ecma (ECMA-334) and ISO (ISO/IEC 23270:2018)

Although both started as proprietary languages, both are open source now, both are developed mainly by one big company, the difference is that Microsoft is interested seeing C# on many platforms, while Apple doesn't have that interest.

I think Kotlin, Go, Rust have greater potential to rise in popularity than Swift.