Up to a point, I agree but it certainly doesn't feel to me like paladins and barbarians are lacking in options, plus, if the constitution modifier after say level 10 level is removed from all classes equally, the effect may not be quite so big as it seems.That's one of the traded off points for the added versatility some classes have. It doesn't make sense to me to work on marginalizing the hit point drawback in having taken theses classes. That drawback is there because the higher hit points in other classes are meant to be a significant advantage.
I'm more of a fan of front loading hp, so I prefer to give max hp plus one hit die at level 1. So on the maths front, the fighter with a +2 con bonus would be 34hp shy by level 20 (6 -20-20), rangers and paladins 44hp (6-30-20), the d8 classes would be 35hp (5-20-20), and d6 classes 36hp (4-20-20). It's only really characters with higher con that suffer, likely to be fighters, barbarians, or sorcerers? A Barbarian with 20 con would be 64hp shy.
I'm not convinced that the effort in rebalancing monster damage would be worthwhile. I'd only bother if I had a one size fits all formula. Multiple attacks mount up with poison damage etc.