Sure, you’d fully pay into Spain’s system, for how long until you retired? How old are you?
By your utopian logic, let’s make the whole world open.
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Nope. Reason - the UK and Europe are places where citizens from both are likely to move around, I'm sure a Spaniard or German would just as well come and live here as we would want to move there. However make that, say Syria and the UK, how many Brits would move to Syria? Count on one hand, whereas I reckon many from there would want to move here. So it's unbalanced.
What will happen, of course, is we offer more visas to places like India, Pakistan, Indionesia, China, Brazil etc etc to get trade deals, and have an increase of migrants from there, but we won't wish to leave in the opposite direction - at least not the volume that we send to the EU to work/retire. So that's unbalanced...but you voted for it. Why??
EDIT; the first part, I'm just short of retirement age, working as a carer for my mother and I already take one private pension.
My plans on her death were to inherit the house, sell (downsizing) and own two smaller properties, on in the UK and one in Ibiza (Formentera if I could afford); live in the UK long enough not to pay taxes in Spain and keep my state pension increases.
I would need private health insurance over there once the EHIC has gone.
Possibility of letting out my home overseas to gain extra income (I have a Formentera agent on the case).
However, the uncertainty over Brexicide has put a stop to that. Hopefully - worst case - I will be able to rent for 6 months and visit 90 days out of 180. But thtat's your decision, you voted for it.