These treatments work best in conjunction with sunlight, which is unfortunately lacking in salt mines.

Not a terrible idea really: Victorian times were also when the industrial revolution was happening, and unregulated coal plants and factories were belching out unimaginable amounts of pollution into city air. Getting away from that mess and breathing clean air probably was good for you if you had health problems.

The comparison I'd make is between a small section of an inactive salt mine with some people in it, and a "modern" city from a century or two ago.

The damp (salt is hydrophilic) walls of the mine could, over time, act as pretty effective passive filters for microscopic particles in the air.

Meanwhile, the city's air is just loaded with particles from all the coal/wood/etc. being burned as fuel.

There’s a reason the saying goes “back to the salt mines.” It’s just a generally pleasant place that people love to be in.

I believe it's a disservice to science to say something doesn't work when there's not enough data yet. I know of a few cases of mines with many people swearing by them and the research I've read said "inconclusive, needs more research", not that "it doesn't work". There's meta studies available: https://pmc.ncbi.nlm.nih.gov/articles/PMC6435215/