It's only rational if you ignore the secondary effects on others. It could increase community spread and ICU usage and thus increase the risk of death for others who didn't make that choice. The other likely causes of death (heart disease, cancer, and stroke) do not spread.
Statistically, there are more people younger than 90 than older, so I don't dispute your statement, but I don't see your point. Is this to justify them breaking isolation?
My understanding of the parent is that it is rational to break isolation because the likelihood of death from other causes is so high already. If the hypothetical 90 year old lived alone (aside from visiting family), then maybe it might adds up that way. My response is that if the person is a community dweller with other old folks and everyone adopted this individually "rational" choice, then the average risk of death from COVID would be far higher because of the increased likelihood of transmission among the most vulnerable (probably why the parent might have gotten some pushback at the suggestion). (assuming this discussion is about restrictions pre-vaccine). Alternatively, if the hypothetical 90 year old moved in with the family, things might stack things up differently.
From my experience the 85+ year olds were mostly isolating at home, the 0-50 year olds out and about in schools, shops, restaurants etc. I'm not sure you needed regulations on the 85+ at all as most are not party animals and the risk of dying for them probably outweighs potential government fines.
