Galaxies Quench All at Once

It's a surprise if sthing is spoiling those conditions, and doing it quickly. That's not what you expect to have from just the sort of random distribution of conditions that you expect in galaxies. You could keep making stars for zillions of years with the gas supply that we have. Stars burn and die, and you could just keep making new stars for a long time. The issue isn't the supply. It's the conditions to make new stars,. remember, you need cold conditions. And they might slow it down to the point where it doesn't fall into your galaxy any more. So you still need that gas.