The main problems with putting it off till that kind of mileage is severalfold. Firstly, as the sensors deteriorate, the voltage they generate decreases, which causes the computer to interpret the signal as a lean condition. The computer will add more fuel, which will cause it to see the signal it wants, but you are now running rich. Running rich doesn't just waste fuel, it causes carbon build up, which can cause all sorts of other problems. It plugs up the EGR
system (I think that most EGR
faults actually begin life as carbon deposits in the sensor and lines from rich operation). The second problem is the longer the sensors are left in, the greater the risk that they will seize in place, making removal and replacement much more difficult. This makes the removal more labor intensive, and there is always the chance that the may not be removable.
Putting it off may be tempting, but remember, todays cars are actually much cheaper to maintain in consideration of how long parts last and how infrequent major failures are. So having to replace a part every 100,000 miles should not be considered a big deal. We should gladly replace maintenance items such as spark plugs and O2
I would find it unlikely that a bad O2
sensor could cause burnt valves though, the lean reading causes a richer mixture, which actually cools and slows the combustion down, which would make burnt valves less likely. However, enough carbon deposits might cause knocking or pinging, which in theory could cause valve damage, so I wouldn't rule it out, though I think you'd have to have an awful lot of pinging going on to cause that.
The Duratec is a lot less likely to have burnt valves than the older OHV
3.0L engine. That engine had lousy valve seats, and when they wore out, the head would crack and that would cause a leak past the valves, which would chew up the valve and cause it to warp. The Vulcan is a pretty good engine all around, but the heads are lousy. Few high mileage Vulcans can be found with good exhaust valves. They will still seem to run fine, but the performance will be down, and the economy suffers a little. The Duratec is however, more likely to have a valve seat issue than some other DOHC
engines. I think this problems is far more common on the Escape than on a Taurus, the Escape is heavier, and is under more load more of the time, so that means it burns hotter too, and it doesn't get the best cooling due to the extra crowded engine compartment of the Escape (its more crowded than a Taurus).