FB:

Your assertion that the safety of high-country water sources changed after the 50s or 60s due to backpackers (or for any reason at all) may be based only conjecture.

It's my opinion that you are incorrect. I think what changed was attitudes and awareness. Back then, nobody wore bicycle helmets or seat belts, and 60% of the population (or whatever) smoked cigarettes.

There is lots of evidence that bike helmets, seatbelts and not smoking are good for health. There is little or no evidence that treating backcountry water is helpful.

There is an assumption that became deep-seated in 1970s, that technology can improve health and safety. But it's not automatically true.

Here's an alternate explanation that is also based on conjecture:

Somebody once said the "Safe Drinking Water Act" in 1974 was the start of the backcountry water scare.

The law mandated treatment of public (municipal) drinking water derived from surface sources. Certain federal bureaucrat/forest ranger types misinterpreted the law, and signage on federal hiking trails that directed walkers to nearby springs were thereby removed. The scare spread from these misguided actions.

It sounds plausible to me.

In the 1980s, an official at a small, suburban water company that had just completed a high-tech super-expensive treatment plant, told me that, according to extensive daily testing, one could safely (but not legally) drink untreated water straight from his company's reservoirs, which were located in the midst of a densely populated drainage basin. I don't doubt it was a highly biased statement, but I bet he was right.