Although my opinion is less warranted than that of a critic, I believe many see this film through a one sided lens. I don't think 'Bliss' was intended to be a sci/fi about opposing realities and simulations nor do I think this film was intended to be a gritty illustration of the affects of drug addiction.
'Bliss' is specifically neither and both at same time.
Throughout the narrative the theme "ignorance is bliss is constantly raised".
This doesn't just apply Isabel and Greg but the audience as well. We are forced to fill in the gaps ourselves whilst paying attention to both realities. We the audience are given the choice to decide the narrative, we become the curators of this story, forced to interpret the fates of the protagonists for ourselves.
We interpret reality the only way it can make sense to us.