If any past coworker I liked wanted to get lunch I'm up for it. For me it's not yet exceeded 8 years back so I guess I can't demonstrate that yet :). I also get a referral bonus so I do want to refer people!

I don’t think I’ve ever had communication with past coworker after my last day of work there. Maybe once or twice.

Because it is not zero sum and some people are up for this. I wouldn't say no unless it was someone I wanted to avoid. And that is very few people.

You made me laugh though!

We live in a world full of uncertainty. When someone you know is in a current need, helping that person is one way to hedge against the uncertainty of the future. Who knows, one day you may need such help too...

That's why this advice is common. It's important to be clear about the reasons for reaching out.