Losing memory via attrition is certainly a concern. But I think the bigger concern is losing memory via censorship and copyright. For most people, the internet is now google search and a couple of social media companies. Google has been curating search results for a while now. We no longer see what is out there in the internet, but a narrow google approved view of the internet. For example, 10 years ago, google showed us the entire elephant ( more or less ), today, we only get to see the tail or the trunk or whichever part of the elephant google decides to frame. Now all of the social media companies are now curating. And with the loss of neutrality, ISPs will undoubtedly curate.

And then there is the copyright issues where archiving is getting more difficult. Kids today don't know what a great tool google cache was because it's gone. And archiving sites are being attack from news, media, politicians, etc.

In other words, the internet dark age isn't going to be a result of formats getting old ( though that is an issue ). It's going to be a result of us only being allowed to see through the frame that a handful of companies deem appropriate. The dark ages didn't happen because formats got old. The dark ages happened because the powerful decided that we should view the world within a certain narrow religious frame and censored everything else.