Why did Trilobites evolve armor?
Your link that says "proven wrong" includes a hypothesis that Opabinia could have eaten trilobites during their molt, simply that it would not be able to eat through their armor (all the better reason to have armor).

The best evidence we are likely to have that trilobites had predators is that they have defenses; the fossil record is incomplete, and just because trilobites are well-preserved does not mean their predators must be. Unless other evidence surfaces for an alternative explanation, the simplest reason for why trilobites evolved armor is that trilobites with armor were protected from something and therefore selected for.

Injuries seen in fossil trilobites are similar to injuries from failed predation of modern related species (see for example Bicknell et al., 2018) and seem to be widely reported (Bicknell et al. 2018 cite a variety of other sources).



Bicknell, R. D., Pates, S., &amp; Botton, M. L. (2018). Abnormal xiphosurids, with possible application to Cambrian trilobites. Palaeontologia Electronica, 21(2), 1-17.