[nnevod]

To fog up, it has to pass water molecules, which are way larger than gas molecules. I've seen only a few IGUs fog up, all of them had clearly visible damage, and they are ubiquitous here, with many 20+ years old. And the IGUs themselves aren't very expensive either (unless they're over 1sqm individually), frames indeed are.

[datadrivenangel]

My parents redid their home with a dud batch from a company that offered 30 years guarantees. Company was out of business within 10 years and 20 years later ~60% of them are fogged. Still very energy efficient.

[WalterBright]

The seals on them degrade and leak.

I've had a lot of them fog up with no damage.