Personally I think each situation is different. ME/CFS and fibromyalgia have that fluctuating and debilitating wave of cognitive symptoms that make a person 'unavailabe' for proper bonding with a child. I would not say that for someone who is deaf for example or who had a leg amputated. There are unfortunately many healthy parents who are incapable of offering proper parenting, but I do not see why it would be wise for an ill person to be an 'unavailable' parent simply because some healthy people are equally bad. The problem is also the uncertainty of the illness: some people will improve over time, some people will remain stable, some people will deteriorate. It is really a bet as I do not think there is a way to predict the evolution of these illnesses.
Regarding the fact that in 20 years' time things will be better in terms of medical care, I doubt and, at the same time, I would love to be totally wrong. I fell ill almost 27 years ago and if things might have improved in terms of diagnosis, I do not really see any fundamental improvement in terms of treatment or cure. If there was, this very forum would not exist. It is more than clear that we are all figuring out the best supplements to make life with these illnesses slightly more bearable. 40 years ago, Nixon announced that cancer would be cured in 5 years. Despite billions spent, for many cancers, we are still using the treatment protocols of the 70's. Likewise for HIV. Unfortunately, ME/CFS and fibromyalgia do not receive that level of funding. Even for an illness like MS that is probably closer to ME/CFS, despite the fact that it has been acknowledged as a auto-immune disease for 30(?) years now, there is still no cure.
About the issue of epigenetics, it seems to be quite a novel concept in ME/CFS and fibromyalgia. It might take decades before a way to change the expression of a gene is found.
I truly understand that people want to have a normal life and have children like anybody else. I have no doubt that people have the best intentions for their children and deeply love them. What I am just saying here is to imagine the situation from a child's point of view as there is a real risk he/she might become ill one day and/or he might have an 'unavailable' parent. And the context is crucial, such as the socio-economic situation of the family (large family? Integrated or not in a small community?) and the medical status (a parent completely bedbound or someone stable who can work part-time?) There is no right or wrong answer.
Preparing yourself for the better is uplifting and feels good now. Preparing for the worst helps to live with yourself if the worst was to happen.