Oh I understand RE alright. Its an idealised mechanism which only works on a level playing field.
Imagine a market with no health insurers. Then three insurers arrive. Their market shares should be distributed roughly equally and with the same healthyness distribution that you mention. If any of them were cheaper/more efficient, they might pick up a higher proportion of customers.
In reality we started with one insurer who already had 100% of the market.
Then, newcomers arrive - what happens to the existing insurer ? Do customers leave in equal proportion across all ages to the newcomers ?
I doubt it.
Older, less healthier, customers who may have been with them for decades tend to stay, whereas younger ones move.
In addition, first-time customers look at the health providers products and decide VHI is too expensive and they join Bupa/Vivas.
It'd be interesting to look at the distribution of first timers in each health-insurer, and ask why they do not join VHI.
The upshot of all this is that the market is skewed because for years there was only one insurer. Effectively,the 'unhealthier' portion stay with VHI.
Over time, this effect should lessen and the other insurers should eventually have a similar distribution to VHI. But only IF VHI make themselves more competitive and stop using RE as an excuse not to become more efficient.
There are lots of factors affecting the type of market share that an insurer has.
Lumping them all under 'cherry-picking' does not help to understand those factors.