[Chyzwar]

Lots of years of experience prove nothing. If someone was working for 10 years in Java backend apps, using outdated tech and in an environment that delivered value is secondary to risk, he/she is not senior.

3 years is minimum for getting close to being competent. I would argue that before that you will deliver very little.

[AstralStorm]

They are a senior engineer... in their specific scope.

These levels are not exactly transferable necessarily.

You should be able to trust that an experienced developer like this would be able to adapt to newer methods and API.

[JustSomeNobody]

There is absolutely no reason to believe that someone with 10 years Java on the backend can't go do, say, .Net at an equivalent level. It wouldn't take long to pick up the syntax, libraries and tooling.

If you take language out of it, the experience is easily transferable.

The problem is everyone thinks their project/product is special and they're inventing something completely new. You're not. Sorry. This is software where everything old is new again.

[mywittyname]

Your experience is not limited to what you do at your job. My previous job was working on a 10+ year old Java backend app and my current job is doing data mining and ML (which I was qualified for and hit the ground running). There is almost no overlap between the positions, yet I've performed well enough in both roles.

One should not expect that people use every skill they have at every job they do.