Not really a problem one would be aware of. Double glazing is the default since 50 years at least in Germany, so the manufacturing processes are quite good.

Still, leakages happen, you recognize that usually by water/steam between the two glass panes. If the windows aren't to old you can just switch out the glass leaving the plastic frames. If they're older (30 yrs) it's more economical to switch out glass and frames.

Almost all my double glazed windows have now failed and I'm looking at replacements, but my fear is that the failures are due to the amount of vibrations this house gets (I'm close to a busy road, every time a lorry goes past I can feel it in some rooms).

From what I can find, the leakage is estimated at < 1% per year. This (German) discussion references and quotes the (German) industry standard DIN EN 1279-3.

https://www.haustechnikdialog.de/Forum/t/65875/Haltbarkeit-A...

Leakage rate is ~1% per year. However the gas isn’t so important you need to replace the window as argon is only up to 16% better than air.

It’s worth it to manufacture windows like this, but unless you’re in the arctic circle I wouldn’t worry about the difference.

There’s been so many window manufacturer lawsuits over this issue that my guess is a manufacturing defect.

Properly constructed, I think these windows are expected to have a useful lifespan of at least 20-30 years. I think the heavier gasses are less likely to leak, so long as the window is kept intact and undamaged.