The answer should be basically none, but I bet it's more like 30-40%. I had a coworker who's mortgage was north of 5%, but didn't want to refi when it was below 3% because they had "been in the house too long and it wasn't worth it". They also were cash strapped, and I pointed out they could get a higher rate and credit back or cash out and STILL pay less in interest, and they just looked at me like a deer in headlights.
I hear what you're saying, but think of the math. I'm trying to see if I can find a chart showing the distrobution of years everyone is into their mortgages, but that's pretty niche so don't know if I can.
If 100% of moertgages were 30 years (they aren't), and the # of new mortgages originated is the same every year (it isn't, its been a been boom the last 3 years), then literally at absolute most, half of all mortgages started 15 years ago or earlier. And even that era was a spike, not a long consistent trend, so many older loans are still lower than today's rates.
Now pile on the reality that any 15 year loan would have cmpleted already (or would be so close the cost of a refi is nonsensical) and that the vast majority of today's loans actually originated within the last decade, and the % of people that even have the opportunity to be a bad decision maker like your coworker is small.