What level of evidence do you require for those explanations? Because all three observations you mention have credible theories, some even have partial proofs.
nobody can explain why eyesight is getting drastically worse
The prevailing theory has to do with lack of exercise of the eyeball, i.e. children spend too much time indoors locked at a single viewing depth; children who spend more time in the outdoors tend to have better eyesight. In China they're running broad studies which seem to indicate that children's eyesight stops deteriorating when exposed to more bright daylight: https://clinicaltrials.gov/ct2/show/NCT05156190
Obesity in the developed world is a big unknown
Sugary/unbalanced foods and sedentary lifestyles? The proofs we have go as far to document the entire systemic changes that ultimately lead to insulin resistance: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6471380/
sperm counts are going down
There's suspected links to endocrine-disruptive pesticides, but FAFAIK no proven mechanisms.
Even if the two you offer theories for are correct, it doesn't really apply to my overall point which is: Big health shifts have happened in unforeseeable ways before and we have no reason to think there won't be any more of them.
Also wearing glasses when they're not needed. Your eyes will adapt to the accommodative stress presented. If you look at a screen all day, you'll end up with a power of roughly -2 assuming you don't wear any glasses looking at your screen, since your eyes have adapted to treat that 2 feet as optical infinity so it can remain in focus without any accommodation needed. If you then go ahead and wear prescribed glasses for close-work where they are not needed, you again restart the process.
Interestingly it has been found that only periphery matters for this process, that's how those fancy glasses like Hoya Miyosmart work, by intentionally defocusing the periphery.
nobody can explain why eyesight is getting drastically worse
The prevailing theory has to do with lack of exercise of the eyeball, i.e. children spend too much time indoors locked at a single viewing depth; children who spend more time in the outdoors tend to have better eyesight. In China they're running broad studies which seem to indicate that children's eyesight stops deteriorating when exposed to more bright daylight: https://clinicaltrials.gov/ct2/show/NCT05156190
Obesity in the developed world is a big unknown
Sugary/unbalanced foods and sedentary lifestyles? The proofs we have go as far to document the entire systemic changes that ultimately lead to insulin resistance: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6471380/
sperm counts are going down
There's suspected links to endocrine-disruptive pesticides, but FAFAIK no proven mechanisms.