That would require a view that Existence and Suffering are absolute synonyms. But last I checked, Schopenhauer and Mainländer don’t exactly enjoy a notable amount of philosophical following these days. (Thank the heavens.)
I think the issue with this kind of claim, honestly, is that they think the only rational thing anyone can focus on is the infinite future. In the case of Charlton et al., that would be eternity in Jesus’s presence (is he supposed to be some kind of bliss elemental?). I’m pretty sure the Jews would contest that kind of focus, and they often hold to an afterlife existing, even if one that’s mostly infinite dreaming or thereabouts (i.e. Sheol). Not fully conscious, outside of Zoroastrian influence.