I think that the issue is that some operators failed to take precautions against ‘fungal rot’ and inspect the aircraft, to such an extent that they were ‘unrepairable’.
I think the answer lies right there in that statement.
The kind of operator that routinely ignores the maintenance schedule and fails to perform the simple tasks that prevent microbiological activity damaging their aircraft beyond economic repair is not going to use any new method of dealing with microbiological contamination either.
The overwhelming majority of operators who comply with the regulations and follow their Approved Maintenance Programme will continue with their present relatively inexpensive precautions against funghi growth and other problems - regular water drain checks, routine "Biobor" treatments and of course, the heavy maintenance tank entry tasks that inspect the internal tank structure for much more than mere evidence of microbiological activity.
why aren't aircraft tanks coated internally, with some type of epoxy-style coating to prevent corrosion issues?
Because we'd have to remove it every check cycle to inspect the structure for tiny cracks and those forms of corrosion that occur naturally over time within the structure of the metal. Some aircraft do have removable bladder tanks and these bring their own problems. Keeps us in work. Fixing things is our job. Its what we do.