I have a brother who lives in Spain who had the choice of giving up his British nationality to get better tax deals. He did, as did his wife.
Denmark has just recently changed their laws on dual nationality, to accommodate the number of Danes who move abroad for work, then do not want to give up their birthright.
Like I said in an earlier post, it's whether you believe in multilateral or unilateral disarmament.
I believe that EU countries will do the right thing, so why not have the UK start it off, and give rights to EU citizens living here. There could always be a time limit; only to those who have lived here over 5 or 10 years.
Someone has to start being kind instead of suspicious all the time.
Do you have any reason to suppose the French or Spanish will treat expats badly?