It should also be noted that despite operating the rides, most technicians have little knowledge about the actual hardware itself.
They know what to check, they now what to look for, but mostly from experience or the manufacturers guidelines. Most technicians do not know if a certain axis from a certain manufacturer is more likely to fail if another one. They do not have the time to go above and beyond for regular maintenance, but keep to what the manufacturer states or the parks policies. The manufacturer designed the parts, they now exactly what kind of stress every single bolt, every single piece of metal can take. And based on that knowledge, they hand out maintenance guidelines to the parks.
It should also be noted, that not every park sticks to the manufacturers guidelines. Some go the extra mile and inspect more than they have to, and change parts more often than deemed necessary by the manufacturer. Europa-Park comes to mind here, but also Djurs Sommerland is a strong candidate for this kind of park. Most parks will just do the minimum what they have to, for the ride being deemed safe. This might sound bad, but it isn't. Rollercoasters are designed way stronger than they need to, and the maintenance guidelines usually reflect what the ride could take if it wasn't overspeced. No manufacturer wants to be in the news when a part fails, even though all necessary maintenance work has been conducted.
It is still possible, that the part just broke because it was never even expected to be used as long as it did, I can't rule it out at this point. It could be a design flaw in the ride, which was not detected anywhere*. The latter option being unlikely, considering the decades the ride operated without any incident.
Last option is that their maintenance was not up to standard, which sounds most likely to me right now. However, they will be a long investigation, which will yield an answer.
*an example would be original Mindbender (i believe, could also be Quimera). With the wheel assembly being located on both sides of the train instead of a central axis as most wooden coasters use, the actual track width has to vary a tiny bit over the course of the ride. With one fixed axis for both wheel-assemblies, the distance between them remains the same at every point in time. But with individual setups, the distance will shrink by a few mm during a curve. Turned out that this was not accounted for during the track construction, which caused immense stress on the wheel-assembly every time the train passed affected track pieces. This will eventually lead to a (partial) derailment.