C++/CLI was just one example.
ODP.NET, EF 6 UI designers, WCF, WebForms, Forms/WPF UI designers [0], and plenty of component libraries are still not .NET Core compatible.
[0] - It remains to be seen if those designers will be shown at BUILD.
The chances are that either (the hypothetical) you would either want to leave the company that is still focused on those technologies or the company would leave you long before your career is over. The last thing a person should want is to have on their resume in two years that their experience is any of those technologies.
There are still companies today using Visual Studio 2008 to support legacy ruggedized Windows CE devices. I have five years of experience with them and still get calls to develop for them today. I don’t go near those types of jobs.
The only ones onsite have more to offer than programming language knowlege, like domain business, architecture design, foreign languages.
Employees that want to stay employable should focus on being T-shaped ones.