First of all, these weren't lost for mankind. They were still stored, waiting to be discovered in scientific sense, I mean by this, waiting until scientists discovered their real scientific potential.
I think all major museums have sort of 'lost' or 'somewhat forgotten' collections, some of them properly stored, some still left in the boxes or wrapped in newspaper from the day they received the collection in many years ago or excavated them.
The main reason that these things are able to happen is that museums are majorly understaffed, with only one or even without a true collection manager for most of the time millions of specimens.
and without skilled technicians to prep specimens.
The only way this is going to change is when the entire society is going to talk to the politicians to make more money available for collection management in museums and depositories (next to making more money available for researchers to become employed and not fired after the have studied for years to build their expertise).
But again, as long as they do not decay and stay inside the walls of the museums, these 'forgotten' parts of the collections are still better preserved in the long term than in private collections or museums.
The last decades, most museologists (this is not the paleontologists) have found plastic animals better for display than the real fossils. This is something all professional paleontologists dislike, but they are not the ones that decide about exhibitions and display pieces, that is the museology dept and the directors. But it will come back, in 10 years or so, all people will dislike the plastic stuff and we will return to real fossils (except for type series, these belong into the vault).
The good thing about the rediscovered insect collection is that only nowadays we are able to see the real scientific importance of that particular collection. 100 years ago we wouldn't be capable of prepping them correctly or getting SEM images etc.
I second all of this. Also, most museum staff looking after local collections are not geologists, or even scientists, and have no idea of the importance of the specimens in their care. It's not their fault - the problem is understaffing, and the result that museums usually have to hire museum studies graduates rather than a host of specialists.
On the plus side, there are lots of stories like this one, of new discoveries in old collections. A beautiful, intelligent and talented palaeontologist of my acquiantance (she's reading over my shoulder, yes) was doing a survey of northwest museums geology collections, and found a pair of lost holotypes of a Carboniferous scorpion and crustacean at Rochdale, and an entirely new species of sponge in Keswick. Both are now written up and in press - papers should be out in the autumn.
The worst that can happen is that a small museum closes, and the specimens are either lost, neglected or even disposed of. It really shouldn't happen; there should be legal arrangements for the collection to go to either a local major museum, or a national. It's worth making sure, though, if you know of a local museum that is closing... sometimes after the curators have been made redundant, the powers that be don't appreciate the importance of adding to their troubles with the complication of making sure the collection is safe.