In my town, there are approximately 12 Durastar Luminaires out there.  1 2000, and 11 3000s.  The 2000 is 100W and 9 of the 11 3000s are 250W and 2 of the 11 are 400w.  All of them were manufactured around 1997-1999, so they have lasted at least 16-18 years.  However, now Ameren Illinois is at the stage of removing and replacing them as it comes time to re-lamp them.  One of the 250W Durastar 3000s have since been replaced with a 2014 M250R2.  I will admit that these have outlasted themselves, nobody really thought they would last as long as they did.  I remember in 1997, many power utilities received "samples" from Thomas & Betts to try and increase sales, and introduce them to the NEW fiberglass and modular construction.  Needless to say, with some utilities, they didn't really take-off.  With others they did.  I was told Ameren Illinois's predecessor, Illinois Power received 15-100w, 20-250w, 15-400w.  They never placed a further order, and now out of the 50, only 12 exist, soon to be 11.  IMO, they were a little easier to work on than other luminaires.  Much like POWR/DOOR, and PowerPad luminaires.  With luminaires like the Cooper OVX and OVZ (which I might add are on their way out here as well, being replaced by GE's M400R3s and M250R2s), you had to remove the wire, and wire up the new luminaire. See Sister Topic coming soon.

Do they replace fixtures that are past a specified age? Like if the fixture is 15 years old, replace the whole fixture when the bulb burns out?

I think that is what they do.  As for Cobra's, they open it up, and it it looks like crap inside, they will replace it (burnt ballast, burnt capacitor, burnt ignitor).  There are still quite a few GE M250R2s from 1989-1990 that shine quite brightly, then there are a few of the same that only reach 60% of their brightness, 1 with 40%, but they have failed to replace them for now.  The NEMAs just get replaced if they aren't GE 201SA HPS heads.

Maybe they had found that some were structurally deficient (fiberglass breaking down) so they decided it would be best to replace them ASAP to avoid a public safety issue.

That is possible.  Fiberglass laminate will eventually come loose

Around here they had always kept the MV lights unless they didn't light with a new lamp or got damaged. With the HPS, they do the same, but any remaining mercs get replaced as they fail.

Well, the D3000 that I thought would be replaced either has an ignitor that is going bad (one of those exterior encapsulated ignitors), or it was relamped.  It is shining now.  I guess that unless the ballast/capacitor/ignitor goes bad on a fixture, they keep them hanging.  Then again, this is in a church parking lot, and they may not have wanted to pay for a new fixture.  Ameren Illinois charges a fee for the kwh use (estimated), a rental fee (they make their money back quickly and turn profits)

If they pay a rental fee, that would cover the cost of all maintenance, including new fixtures.

The Durastar 3000 in question has been replaced with an M-400R3