I think the argument isn’t that there literally cannot be heaven without hell, but that our perception of heaven’s… heavenliness depends on our perception of hell’s hellaciousness.
If you do an experiment in which you put a 20 watt light next to a 60 watt light, and you ask people if one of them is bright, they’ll say yes, the 60 watt bulb is. Then if you put a 60 watt bulb next to a 120 watt bulb and you ask them if the 60 watt bulb is bright, they say no, of course not. The one next to it is.
Our perception of nearly everything depends on a relative rather than an absolute scale. (We use absolutes to try to calibrate and communicate sometimes, but our subjective sense is still determined relative to the environment around us.)
Even something as objective as temperature depends on the context you put it in. When I lived in Vermont, the first day the temperature got above freezing and the snow started to melt was a glorious, warm day, even at just 33 F. Now, as a southern Californian, the second it dips below 60 everyone’s bundling up and staying inside, myself included.
So sure, you can have heaven without hell, or day without night, or happiness without sadness. But you’d have a much harder time perceiving each one for what it is without the other.