Languages and specially the tool chain and the way of doing things around them evolve. If you spend like 5 years doing something else and come back to it, you'll find things have changed a lot. You may argue that it's easy to relearn this. But from a job perspective, the guy who has been using the exact same tech stack in the recent years will often be a better bet than a guy who wrote Java in version 1. This is very true for C#. Syntax, design principles, tools, method of deployments all have changed rapidly. Professionally, you want to be as up to date as possible in the area you worked in or you get rusty. You can learn new things on your time for their own utility. Flushing down 40-60 hours a week on a useless piece of knowledge is bad all round to me.
