If they've been there for 10 years, I would try to find what changed in the past few months. Is it an issue at home, or maybe something that happened at work? I feel like you have to find the root cause to fix it. It could be something where they're going through a family illness or a divorce and need some lower complexity tasks for a couple months.

Also, being somewhere for 10 years doesn't make it hard to fire someone. You could fire them for performance issues easily if the stuff coming back is low quality and half done. You can give them metrics to hit on a PIP and if they fail, they're gone. But I wouldn't want to do that until I knew what was going on that caused the change. It could be a temporary thing or an easy fix.