Think it's a case of better safe than sorry, if protection does wane then as you allude to, if the third jab keeps tourists out of hospital then that's a good thing. They will have enough cases of their own than to be taking in tourists!UK Government confirmed they are working on adding 3rd jab status to the NHS app.
But yes it does seem a bit hasty...as you say UK only just getting going on top-up jabs and other countries are even further behind and still on 1st and 2nd jabs. If Spain made that move they'd be effectively shutting their borders for a while, airlines won't fly if they can't sell many seats.
Debatable what impact requiring a 3rd jab would have anyway, 2 jabs significantly reduces a Tourists risk of going to Hospital so that has made sense, but even when 2 jabs wear off there's still some protection. Seems a bit overkill to me,
As we are all tourists on here - save for those lucky enough to live on Ibiza - all I would ask for is consistency throughout Europe and to be given plenty of notice for changes.
New infections seem to be on the rise everywhere regardless of the vax but I'm not too concerned about any rule changes for my trip to the Costa del Sol next week. Famous last words.....