The problem is not whether it is legal to use Swift in a 3rd party environment, but the lack of support in the 3rd party eco-system per se means the attempt would vanish, in time.
I believe most of us who actually use Objective-C choose it not because the language itself is designed to be simple and elegant, (and yes the language is simple and elegant,) but because the Cocoa and Cocoa-Touch are well supported by Apple. No offence to the effort outside of APPL, but it is not enough to support a full-fledged eco-system with all 3rd-party F/OSS written in Objective-C that is compatible with what Apple currently uses in order to build reusable components. GNUStep has existed for so many years, and Étoilé for years as well, but I did not see emergence of adoption in community comparable to that of other popular F/OSS frameworks, e.g. Mono and Qt. And I do not see how this issue could be circumvented, unless some big player shows up and adopt any of these projects, like what Google did to Android.
I believe most of us who actually use Objective-C choose it not because the language itself is designed to be simple and elegant, (and yes the language is simple and elegant,) but because the Cocoa and Cocoa-Touch are well supported by Apple. No offence to the effort outside of APPL, but it is not enough to support a full-fledged eco-system with all 3rd-party F/OSS written in Objective-C that is compatible with what Apple currently uses in order to build reusable components. GNUStep has existed for so many years, and Étoilé for years as well, but I did not see emergence of adoption in community comparable to that of other popular F/OSS frameworks, e.g. Mono and Qt. And I do not see how this issue could be circumvented, unless some big player shows up and adopt any of these projects, like what Google did to Android.