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The lifetime charts that were up here on FR when this story broke were based upon 250 hours per year of operation. Most the F-15's have far exceeded that. If you look a calendar years may be but in flight time I see they made their numbers.

From one of your links:

A thinning of the longeron at a key stress point - possibly due to a manufacturing defect - may be the root cause of the mishap and the cracks found in the eight aircraft. "More than likely it is a manufacturing issue and we have pulled all the Boeing material discrepancy reports," the group commander said. "So far, we have not been able to isolate it to a particular production run or series. The cracks show up in aircraft as old as 1978 and as new as 1985." Recurring checks had not called for a review of the area in the past. "It was a 25,000 to 100,000-hour part," the colonel stressed. "So it was not included in our depot and phase inspections. It was designed to significantly outlast the aircraft." "

So there maybe a manufacturing defect but only supposition at this point.

The aircraft that broke up causing the fleet grounding was built in 1980 and if it did average 250 flight hours per year, then it was at 25,000 flight hours, but nowhere near 100,000 flight hours.

Here's one take on F-15 lifetime:

F-15 Eagle Service Life

The F-15 initial operational requirement was for a service life of 4,000 hours. Testing completed in 1973 demonstrated that the F-15 could sustain 16,000 hours of flight. Subsequently operational use was more severely stressful than the original design specification. With an average usage of 270 aircraft flight hours per year, by the early 1990s the F-15C fleet was approaching its service-design-life limit of 4,000 flight hours. Following successful airframe structural testing, the F-15C was extended to an 8,000-hour service life limit. An 8,000-hour service limit provides current levels of F-15Cs through 2010. The F-22 program was initially justified on the basis of an 8,000 flight hour life projection for the F-15. This was consistent with the projected lifespan of the most severely stressed F-15Cs, which have averaged 85% of flight hours in stressful air-to-air missions, versus the 48% in the original design specification.
Full-scale fatigue testing between 1988 and 1994 ended with a demonstration of over 7,600 flight hours for the most severely used aircraft, and in excess of 12,000 hours on the remainder of the fleet. A 10,000-hour service limit would provide F-15Cs to 2020, while a 12,000-hour service life extends the F-15Cs to the year 2030. The APG-63 radar, F100-PW-100 engines, and structure upgrades are mandatory. The USAF cannot expect to fly the F-15C to 2014, or beyond, without replacing these subsystems. The total cost of the three retrofits would be under $3 billion. The upgrades would dramatically reduce the 18 percent breakrate prevalent in the mid-1990s, and extend the F-15C service life well beyond 2014.

The F-15E was built with a 15,000 hour airframe life in mind from the start. It is also heavier and less maneuverable as a result.

We don't need to repair the F-15As, but the C models will be around for a long time yet. Service Life Extension Programs are not unusual.

Other aircraft have undergone life extension airframe modifications. The B-52s that are still flying are not flying on their original upper wing skins. A-10As have been rebuilt at AMARC. F/A-18As and Bs have hand their center barrel sections replaced.

We need F-22s, and we also need F-15s. It is not an either/or situation, but rather a question of what is the right mix today, tomorrow, and in ten years.

For that matter, the same goes for the F-16. We will be retiring old F-16s as F-35s come online, but we will not get rid of all the F-16s by any means.

How long did the F-100, F-4, and A-7 soldier on with ANGs after they were replaced by more modern aircraft? The last The F-100 retired in 1979. The A-7 was retired in 1991. The last F-4 was retired in 1996, and still flies today with the Japan Self Defense Force. The B-52 (as a type) may see 100 years! It celebrated 50 years in 2002.

If it takes $250,000 to make an F-15C airframe last another 2,000 hours, it's money well spent compared to the couple of million we're spending (and have spent) equipping them with JHMCS, AESA radar, Link 16 data links, and GPS/Ring Laser INS that makes them very capable aircraft in the future.

Absolute front line first day? No. But very useful as second line fighters in domestic intercept and lower level conflicts.

I was looking for an article that gave the flight hours of the mishap F-15 that forced this grounding, and here is finally is. Sounds to me like this one definately did not make it's numbers.

Manufacturing defects caused cracks that downed USAF F-15

Fatigue cracks started by manufacturing defects in a fuselage longeron caused the in-flight break-up of a Boeing F-15C Eagle on 2 November 2007, the US Air Force accident investigation has concluded.

Longeron cracks have been found in another nine F-15Cs, and manufacturing defects that could potentially cause fatigue cracking have been detected in a further 182 F-15A-Ds. All of these aircraft remain grounded.

Examination of the wreckage of the crashed F-15 revealed the right upper longeron, a critical load-carrying component in the forward fuselage, failed because of a fatigue crack that formed where the metal was thinner than specified in the blueprint.

Instead of being the specified 0.090-0.110in (2.3-2.8mm) thick, the flat top, or web, of the aluminium longeron that failed was as thin as 0.039in – less than a millimetre - in the area when the fatigue crack formed.

The thinning was caused when the longeron was machined by McDonnell Douglas during production of the aircraft, which was delivered in 1982. Similar manufacturing defects - undercuts, ridges or surface roughness that could potentially cause stress concentrations – have been detected in upper longerons in 40% of fleet.

This was by far the worst thinning of a web discovered by the fleet-wide inspections that followed the crash, says Maj Gen Thomas Owen, commander of Warner Robins Air Logistics Center, the USAF’s F-15 depot.

The accident aircraft had accumulated 5,700 flight hours, says Owen, or 11,000 “equivalent spectrum hours” taking in account the additional fatigue stress imposed by high-G manoeuvring. But the longeron was projected to last 31,000h, well past the life of the aircraft, he says.

In the 2 November in-flight break-up, the cockpit separated at canted fuselage station 377, where the forward and aft sections of the two-piece upper longerons are spliced. The fatigue crack had formed just forward of the splice, in an undercut produced when a sloping transition was machined in the underside of the longeron.

Investigators say the crack started on the underside of the longeron, working its way upwards and outwards through the over-thin web until it reached the outer “posts”, which carry the bulk of the forward fuselage loads.