I have seen tanks with sand bed 20 years old.
As far as OTS there are so many theories, I have seen articles, talks at IMAC and MACNA etc.. I have seen heavy metals blamed because rocks can bind some, Sandbeds, Live rock getting chocked to the point it does not work, neglect, plumbing gets sponges, bacteria, tube worms or what ever in it slowing flow down, diversity disappears after a while. etc.. But I have seen people replace parts of the bed, Live rock etc and still get it.
It does exist, I have been in the reef hobby since the late 90's and have seen it.
I have seen tanks 20 years old and nothing changed, one day the tank looks great and next day it is on a unrecoverable decline. I think it is allot of things and maybe a combination of it all. I think the time frame depends on how well you took care of the tank, if you let it go for a period.
I do not think we see it much any more because people are in and out of the hobby so fast. How many 20 year old tanks are there? How many 15 year old tanks are there?
Some people do get tanks to last allot longer or indefinably but that does not mean it does not exist. Lots of diseases out there that affect humans and not everyone get them but it does not mean it does not exist.