I had to read that sentence a couple of times but I think it’s just poorly worded. He’s not saying that the person is old, he’s saying that the reason for wanting to fire them “you are not doing a great job”, is a “plain old” reason

Im fairly certain he means 'plain old not doing a great job'.

I'm 99.9% sure OP the author didn't mean that people should be fired for being old. I agree that the phrasing is open to misinterpretation, but I read it as "plain-old not doing a great job"

You read the comment wrong.

What he meant wasnt age.

"or just plain old [saying] 'you arent doing great job'"