Why is letting sparkling wine sit open to 'breathe' usually a bad idea?

Answer: The bubbles escape and it goes flat

The bubbles escape and it goes flat ✓ — Correct! 'Breathing' means leaving wine exposed to air, which can soften a young red. But that same exposure lets dissolved carbon dioxide escape from sparkling wine, and the bubbles are most of the point: they carry aroma upward and give the crisp, prickly texture. Serve it cold, straight from the bottle, and let it open in the glass instead.

The foam makes it taste smoother — Not quite. Foam does feel soft on the tongue, but it is a sign that gas is leaving the liquid. As the mousse collapses the wine turns slack and syrupy rather than smoother, which is why flat champagne tastes oddly sweet.

The aromas need oxygen to open up — This is true for many still wines: air helps a tight young red release its aromatics. Sparkling wine works the other way round, because the rising bubbles already ferry aroma compounds to your nose, and losing them costs more than the air gains.