Earlier quoted context omitted.
"namely, a senior employee likely has a lot of his/her own personal overhead to cover, so salary requirements will be understandably greater" I wouldn't assume that, once the mortgage is paid off and kids have left home the cost of living plummets. In fact there could be a real cost advantage if nobody else is offering them a job.
That's a big and dangerous assumption. Take the Bay Area...many people living here now that will likely never be able to buy a home if prices stay even close to where they are (let alone ever pay it off if they did). So then they have large mortgages or high rents. There's also increased risk of major medical expenses, the baggage of paying for things like divorces, college, etc.
Right now you can assume those with high mortgages are in their twenties and thirties, someone in their fifties probably sorted their housing problem twenty five years ago.