I just realised why some people don't use/upgrade their trinkets

It is simple: they have not experienced the game without them. We who started before S4 have warding imprinted in our DNA because we have lived at the age of the ward wars, where the support would buy a sightstone and wards only and each bush had a large pile of ward corpses. Trinkets were bought with every support's blood, sweat and tears, it has become natural for us to use them. It is the same as a modern working person compared to a slave from 500 BC: for the modern person, 8 hours of work might seem too much, but for the slave it is better than all of their dreams combined.