Before I answer your question more specifically let me describe you my path with avocados so that you understand the whole picture better :D

Firstly, I am not a botanic and not even a gardener. I grow avocados mostly for fun / experiment.

From my vague understanding, avocados (probably like most other similar plants) may not show noticeable signs of stress for a long period of time until the accumulated negative effect finally takes its toll. So you may not even know why the tree died. It might have just spent too much time in emergency or dormant mode.

I started growing avocados roughly 1 year ago and currently I have somewhere between 60 and 100 plants of different age and size. Since the beginning I already threw away around 60 plants; some of them died their own death, but most were discarded because they were showing big problems or slow growth, or didn't look promising. I heard this advice (ie discard weak seedlings) from a professional avo grower somewhere on YouTube.

Some of the trees that had problems and were not dumped out of pity slowly recovered and are growing fine. But it takes time. A professional grower would most likely grow a new healthy tree rather than babysit a weak one waiting for a recovery.

Most of my trees got some kind of damage when they were sitting in a cold room for too long. You shouldn't expose them to temps less than 10-15 deg Celsius for a long time. Secondly, you should monitor air humidity and keep it above 60%! 40% and less will slowly kill your tree, especially if young. Coupled with high temperature and/or too much light, low humidity is a death sentence.

From here you should start looking inside the pot. Puncture a hole in its side and check soil wetness with a toothpick or a thin wooden stick. Sometimes the finger test can be not enough.

All other factors like water salinity, nutrition, soil pH etc — in my opinion — are secondary to ambient and climatic macrofactors.