Yes I agree that they should have seen this coming. Many did, actually. Same in France too. Many Brits (who didn't vote leave and are furious about it) have been complaining that they now have to apply for residency and integrate fully into the French system in order to be able to live there all year, (which is leaving them worse off financially and that's why they are reluctant to do it.) Even though France has made it very easy for them to do so and hasn't thrown anyone out to my knowledge, they now need to declare their income and pay French taxes on it. Trust me, no-one likes paying French taxes if they can help it.
They should have been doing this anyway - it's just that no-one ever checked. Many Brits in France and Spain have habitually stayed for more than six months a year (some barely leave and do not have a UK home to go to) and therefore should have been filing tax returns there, even if all their income came from the UK via pensions or working remotely.
Lots pick up casual work wherever they can, often for cash in hand and don't declare it either, so they have been able to remain beneath the radar for a long time, returning to the UK only when they need treatment on the NHS.
In my experience most of them are not so stupid (maybe in Spain but not in France) that they didn't realise the consequences of Brexit on their lifestyles - hence why those in France were so vociferously against it.
I know British micro-entrepreneurs who don't declare their full earnings in France to keep it below the threshold for higher taxes. But in order to show they can earn enough to stay they now need to declare more and therefore pay tax on more. They are thrilled, as you can imagine.
Things have finally caught up with them. They want the more relaxed work/life balance of France affords without having to pay dearly into the French system to get it.
Those who are so stupid to not have realised, perhaps were lulled into a false sense of security that the EU would reciprocate with 180/365 the same as the UK has offered EU citizens. Personally if I were negotiating this I would have insisted on reciprocity, country by country. I know we can't ask for a blanket exemption from the Schengen rules as a Third Country but why we ever felt we should give 180/365 when we wouldn't get it back is a mystery to me, and I think it should absolutely NOT have been offered to EU nationals.