There’s absolutely no lack of evidence. We know it’s real. We just haven’t catalogued that evidence very well.
That doesn’t look likely, but also not impossible. That’s why we need the study.
Also, predator-prey systems (https://en.wikipedia.org/wiki/Lotka–Volterra_equations) can have fairly large cycles with huge population swings. That makes verifying that there really is a decline not _that_ easy. You can’t just repeat an insect count done 30 years ago and compare the numbers.
https://www.theguardian.com/environment/2020/feb/12/car-spla...
> The survey of insects hitting car windscreens in rural Denmark used data collected every summer from 1997 to 2017 and found an 80% decline in abundance. It also found a parallel decline in the number of swallows and martins, birds that live on insects
Has the evidence improved recently? About 3 years ago there were a few papers, including a meta analysis (-ish), which were suggestive but also noted the lack of longitudinal studies, and their lack of geographical diversity (Germany and UK as I recall).
The consensus was that everyone was certain there was a serious issue, but it was nearly impossible to usefully (even broadly) characterise the issues.