My impression is that economists generally thought the Swiss central bank would keep the peg, because why wouldn't they? There's no economic law stopping them and it was obviously good for the Swiss economy.

The fear of central banks with large balance sheets is a powerful political force, one that economists don't really appreciate. The rational thing would be to keep the peg (or even better, target inflation or NGDP) but an unfortunate majority of politicians and voters are fearful of "printing money". Just as central banks around the world keep making the mistake of allowing falls in the inflation rate during a recession, economists keep making the mistake of assuming the next central bank will listen to them.