This post that was quoted has been deleted.
Yes ... not sure where you got that 300.000 figure from - there’s around 384.000 expats in Spain, and as Chrisso says only around a quarter of that number are pensioners, so your argument falls flat there.
On top of that, the people that DO come in as from 1/1/21 will be, as Andy demonstrates, financially better off than a lot of pensioners, and for the first 5 years they’ll need to prove that wealth at three intervals, so I foresee, again, as Andy points out, contributions to Spain’s coffers.
But again, as Chris has mentioned, the UK is not the only country that has pensioners living here. Where I live, the people are all French or Spanish - only one or two Brits here.
Spain is going to be fine. There will be no “special arrangement” for the British. It’s simply not needed.
But I say again to those who ARE coming; please be aware this is NOT England with sunshine. It IS a different country and it may take a little time to adjust to the very different culture and customs - but it is very worthwhile.