How common is it, really, for someone to stay with a company for 20 years? And why do I care about data that won't affect me on either end until, at the least,

How common is it, really, for someone to stay with a company for 20 years? And why do I care about data that won't affect me on either end until, at the least, 2028? Presumably, all these numbers will be very different by then.