[KarenO]

I wonder if seed borne disease factors in.
Everyone and their pup has a little website and is selling seeds these days including from some highly seed borne disease prone areas in the American south for example with no germination testing, no quality control or even the barest hint of such niceties as a phytosanitary certificate. The flashy websites give people false confidence that they are dealing with professional seedhouses but often that is not the case, it’s an unregulated, unlicensed, Uninspected under the kitchen table enterprise charging top dollar for what is possibly dubious quality seeds.
Clearly there are excellent smaller experienced seed sellers who have sold quality seeds for years but there are many newer ones in the last couple of years as well and more all the time.
Combine this with a lot of trading with sometimes unknown or inexperienced sources who may also be saving seed from diseased plants ..
I don’t know this is a factor in the circumstances you are speaking of directly but numerous viral and bacterial diseases (including rapidly lethal wilts)can be seed borne.
Reusing old pots support sticks etc without cleaning can also be a source of fungus and pathogens.
Healthy plants don’t just die. I see planting medium blamed a lot, especially one certain brand but perhaps it’s something else.if the opportunity exists for a professional diagnosis of the “ cause of death” that would be very helpful.
KarenO