Let's say SSDs have an annualized failure rate of 1%. (The real figures are from 0.5% for more reliable ones, 1.5% for less reliable ones). The annualized failure rate for HDDs is a bit higher, but let's give it 1% anyway.

Over 40 years (even with changing disks, upgrading PCs, etc.), the chances of you ending up with your data intact is 0.99^40=0.67. May not seem too bad, but this is just a hardware failure of your main drive we're talking about. When you factor in all the other types of screwups that happen, the annualized failure rate will skyrocket.

So a more conservative version of this quote is that a 20-year old today is very likely to experience data loss over his lifetime.

Thankfully with cloud being ubiquitous at least photos are given one copy in the cloud, but IMO even that is risky. People have different appetites for risk, and that's OK. But over the last 20 years the all of our data has become pretty important. I don't know how many people could stomach the loss of important documents, photos, etc.