We haven’t seen a quarter of known bee species since the 1990s
As suggested by others the number of taxonomists, people who could accurately identify species (many of which require internal dissection, or molecular methods), who are actually working on collecting and identify species has most certainly declined.  In many groups of insects there is at most a handful of experts worldwide who can take specimens to a species-level identification.This is not to say that species richness is not declining, its to say that in the past 3-4 decades Taxonomists have done a poor (some would say terrible) job at describing to the broader world why they are important, and why they require fixed, institutionally-based funding to actually be able to provide the services that would allow us to confidently state that data like these are because of environmental change (again, they very likely are) rather than a lack of experts in the field actually doing basic research.In other words, it is extremely rare that universities (in the US) actually hire what was once known as "alpha-taxonomists", in part this is a reflection of taxonomists inability to sell themselves and adapt to new tools (but note that many have evolved) in part it is a reflection of the (I would argue "immense") short-sightedness of institutions. "We want answers to complex questions!  We've neglected to give scientists time to think deeply, and research over decades to answer those questions.  Oh, we see."