Vanguard 1 became the first satellite to use solar panels as a power source in 1958, when it was launched into an orbit with the following parameters:
654 by 3,969 kilometres (406 mi × 2,466 mi), 134.2 minute elliptical orbit inclined at 34.25 degrees on March 17, 1958
Source, sadly without citation, from Wikipedia.
As you can see, its periapsis is within the tenuous grasp of the exosphere. It was estimated at the time of launch that its orbit would decay over 2000 years, but at the time, it was not known that the exosphere "puffed up" during periods of high solar activity:
[this] caused a significant decrease in its expected lifetime to only about 240 years.
So, if this is correct (and it will only be a rough estimate as orbital decay predictions are notoriously inaccurate), it has already spent nearly a quarter of its total orbital lifetime in orbit.
Yet, the satellite tracking website N2YO (which receives regular TLE updates from current orbital objects) has the satellite currently in a 653km x 3833.5km orbit, inclined 34.3 degrees.
Would we not expect to have a seen a greater decay than what has occurred so far? If these details are correct, its apoapsis has only decayed by 136km since launch, and its periapsis has not shrunk at all, at least not appreciably. This does not appear to match its predicted orbital reentry timeframe of 240 years. What is going on here?