For example, I'm pretty sure multiple decades of little visual aids developed by academic tutors as Java applets just became inaccessible to almost everyone. There was nothing wrong with many of those demonstrations in the content they presented, nor is there anyone responsible for replacing them. That valuable content has effectively now been lost.
There are discussions going on about Firefox 52 today on every relevant technical forum I follow online, and all of them (including this one) have threads showing a variety of other useful content or services that relied on these plugins.
There is no reasonable argument that this was necessary because maintaining support was too difficult. The same browsers eliminating NPAPI support are the ones adding things like WebAssembly.
There is no reasonable argument that this was necessary for security. Most of the plugins were click-to-play in all major browsers already, dramatically reducing the risk associated with them, and more importantly, there is no evidence to suggest that browsers won't suffer from a similar level of security vulnerabilities as they add ever more native functionality to replace what the plugins were used for before.
I realise I'm posting on HN, where the culture and interests are often more about building throwaway web apps and worrying about longevity later, but most of the Web and most of the world isn't like that. There is a price to pay when support for standards that have been around for 20+ years is dropped.
Fear not, my friend. If we accept the premise that everyone should be forced to switch software every few weeks to retain access to useful content, even if doing so means losing access to other content in the process, then much of this time won't be looked back on at all.