I've been in the climbing gear manufacturing business, so I'm not guessing. 6-sigma quality control and all.
"Nope. Both SHOULD be replaced at the same time. But they won't."
This is not clear. If the tether has not ripped ALL of the bar tacks, the biners have not seen a load over 600 pounds. Additionally, in climbing and industrial use it is common practice for the webbing to be damaged or abraded long before the biners are damaged. User inspection is always vital.
"and how much does it cost to get the new webbing sewn in with the break away threads and certified safe? "
I was not suggesting DIY. The webbing by itself could be sold as an assembly for that cost. Yates 'Screamers' sell for $18.00 and are similar. By far, most of the cost is in the biners.
"But it's accepted that seat belts (like a standard tether) can injure you. No one seems to be outfitting millions of cars with break away threads to indicate straining. I just think this is more than what's needed."
Not a good comparison. Seatbelts hold the body in a different manner. If they stretch significantly you strike the dash. A loosening fit could be very dangerous, as the contact points would become dangerous (stomach and neck). Cars are designed with crumple zones to limit g-force. No, not a good comparison. I think.
Maybe it is more than what's needed; it's a forum, so this is just for fun and to make us think. Otherwise, nothing changes.
"And what was the distance of those falls? Source?"
I'm sorry, but this is common knowledge and common sense. OSHA flatly forbids the use of a chest harness where fall potential exists. Ask any construction worker or any rock climber to take a 2-foot fall in a chest harness against a static anchor (not a stretch rope) and they will explain to you how much that will hurt. Don't try it; you will be injured. You will find little discussion of this on the net, because climbers learned chest harnesses where nuts 60 years ago. Thus, up-loaded modern data is lacking.
Chest harness are not acceptable where fall potential exists (like off the deck). Only positioning. Fall Protection
. They give big tickets for that.
Energy absorbing lanyards are required at 6 feet.
To learn about fall energy and shock absorption, rock climbing classes and engineering course work are useful. I have taken short falls (2 feet) against non-stretch anchors in a seat harness, and the impact is truly bone jarring. Climbers have broken 5,000-pound test gear in 3- to 4-foot falls. Ask the climbing instructor; this also is well known and is the reason Yates makes 'screamers.' (Yates invented these for climbers long before OSHA figured it out. Notice that 'Sceamers' are sewn the opposite direction from OSHA tethers; climbers know the bar tacks cause high frequency vibration when tearing that can rattle rock climbing gear and cause biner gates to flutter open, which weakens them. Lots of on-the-cliff fall testing.)