I assume the reason that diabetes (when medically treated) qualifies for a prescription exemption is that the complications from it being untreated would end up costing the N.H.S. a lot more.
The further away something is from us the more abstract it seems. It is all too easy to dismiss the risk of going blind many years in the future as something we do not need to worry about until then.
If I do not take my diabetes meds I will not feel any consequences, it is not really meant to change my life now but prevent it from changing in the future.
While people will not skip medications they need to stay alive, I would guess there are those who may choose to not spend money on medications they will get no immediate benfit from. I thnk it is around a third of all prescriptions issued which are not collected. And some of those who do either skip or reduce doses to save money.
Of course, paying for prescriptions is only an English political issue anyway. They are free for everyone in Wales, Scotland, and Northern Ireland.
Even Labour would not abolish them, but they started a review into extending the long-term conditions which qualify for free prescriptions. But that was immediately dropped by the Coalition and has been dismissed by subsequent Tory governments.
I gues it is just not a big enough issue to most voters for politicians to have to care about. Given a choice, I expect many people would instead vote for a tax cut to take away the existing free prescriptions.