Except in reality this only works for the server side programmers. At least where I am from a server side developer commands a much higher salary than a front end developer. So what is the incentive to specialize in the front end?
Additionally as far as I’ve seen most senior or lead programmers do not specialize. To me this shows that specialization creates a ceiling in your career path. Especially when moving between companies. Many companies do not have separate server and front end roles. So if you specialize you are just limiting your future career options.