Asking an iOS developer to work on Objective-C is asking them to sacrifice their future prospects.
The impact a language has on your career prospects is U-shaped. Knowing new languages is good for your career. Stagnating on an 'old' language is bad (Swift is only 7 years old...). But eventually knowing the old language puts you back in demand again, to port and support old codebases.
The impact a language has on your career prospects is U-shaped. Knowing new languages is good for your career. Stagnating on an 'old' language is bad (Swift is only 7 years old...). But eventually knowing the old language puts you back in demand again, to port and support old codebases.