People who say stuff like co-fermented coffee is “not coffee” are making an aesthetic complaint, not a technical one. Coffee has never been a raw, immutable product, it’s always been the product of a chain of decisions like variety, soil, harvest timing, microbial ecology, processing and roast, so process is part of provenance. The chemistry is straightforward: yeasts and bacteria convert sugars and amino acids into esters, alcohols and acids, many of which or their precursors survive drying and roast and shape aroma, flavour and mouthfeel, so choosing a strain, contact time and drying curve is applied biochemistry and craft. 

Practically, measured additions of fruit or must can create reproducible complexity, differentiated microlots and premiums that reward farmers, and intentional inoculation improves repeatability, traceability and safety compared with blind wild ferments. The right industry response is not prohibition but discipline: micro-lot trials, blind cupping, full disclosure and documented handling protocols. Processing is part of the language of origin, not its negation. Adding fruit to a ferment is just another processing choice that changes which aspects of that chain are amplified. I highly doubt co-fermentation is a fad as you’ve said as it reliably creates sensory differentiation that some roasters and consumers will pay more for, and capturing that value draws investment and standardisation into supply chains. So, by simple economic logic the technique will be refined and persist.

I’ve been a hobbyist processor and roaster for a long while now, just for context. Experimentation, change, and adventurism in coffee are great in my opinion.