Okay, please explain in what sense Python or Rust is proprietary.
It is literally illegal for me to start marketing The TorstenVL Rust Compiler. Because the language is proprietary.
Trademarks are annoying but I can hardly imagine they're what anyone is worried about when picking a language in this context, they're not what's going to cause a language to disappear.
What you can call something and whether you can legally make the thing or have a permissive license to an existing implementation are two completely unrelated things.
For example, you also can't make a C compiler and name it the "microsoft C compiler" due to your lack of trademark right. Does that mean C is also proprietary?
See also: The most famous open source project is trademarked https://www.linuxfoundation.org/legal/trademark-usage
If you still aren't convinced, you are definitely using a different definition of the word proprietary than everyone else.
> Proprietary software is software that grants its creator, publisher, or other rightsholder or rightsholder partner a legal monopoly by modern copyright and intellectual property law to exclude the recipient from freely sharing the software or modifying it, and—in some cases, as is the case with some patent-encumbered and EULA-bound software—from making use of the software on their own, thereby restricting their freedoms.[1]
> Proprietary software is a subset of non-free software, a term defined in contrast to free and open-source software; non-commercial licenses such as CC BY-NC are not deemed proprietary, but are non-free. Proprietary software may either be closed-source software or source-available software.[1][2]
The Python development team, via the LICENSE file in the GitHub repository, tells me
> All Python releases are Open Source (see https://opensource.org for the Open Source Definition). Historically, most, but not all, Python releases have also been GPL-compatible; the table below summarizes the various releases.
This license is also described on Wikipedia at https://en.wikipedia.org/wiki/Python_License .
Similarly, the reference Rust implementation (https://github.com/rust-lang/rust) is licensed under Apache 2.0 and MIT licenses.
In what sense is FOSS software a kind of non-FOSS software?