This advice makes a lot of sense to the affected person after the fact. But when someone's hurting, in that moment, the cannot relate to this advice. Human nature is immensely complex, and hence weird in that way.

OP and his wife have been struggling with the loss of their son, a part of them, for years now. By now they must've heard this advice to move on many times; but it just doesn't feel right to the person experiencing the grief and pain. The people around them, and our community here on HN, needs to provide them with small, actionable steps, so that they can slowly work up from where they are right now, without sacrificing anything or values that they hold dear to their hearts. Asking them to let go is most likely going to be counter-productive.