EHIC isn't insurance and tourists who use it to buy travel insurance that excludes a pre-existing condition are gambling.
EHIC doesn't cover everything but it does mean you will be able to avoid medical bills that will bankrupt you. You can live without the other things that EHIC doesn't cover, such as repatriation. EHIC is the only option for people who cannot get affordable travel insurance. They are already effectively barred from going on holiday to most of the world outside Europe.
So that is a huge loss. What if you get cancer before you retire? Then you won't be able to retire in Spain, but you would have been able to do so if you came from an EU country. Therefore if you are serious about retiring in Spain, you need to move to Ireland as soon as possible in case you get ill before retirement. If you are already ill, your only option is to spend 5 years in Ireland first.If British emigrants with pre existing conditions cannot afford to buy health insurance then they would be a liability to their host country (which they are currently forced to accept because of FOM). Its reasonable for countries to set levels for third country citizens that ensure immigrants are not a liability (and they they learn the national language to a reasonable level).