This isn't new, and certainly isn't some crisis of the last 100 years. Historically only a minority of the planet has ever been lactase persistent into adulthood. It's a well-understood genetic trait, not some product of gut microbes.
What's the separate mechanism? I'm only aware of one set of genes coding for adult lactase production.
Also a possible reason for why people have different taste preferences.
Humans are odd in that we harvest milk for consumption at all beyond our own mother's, which is probably why we're the only species with a notable population with the genetic mutation that allows some of us to digest it – there's no competitive advantage to that mutation in most other mammals, because where would they even obtain milk?
Lactose intolerance is normal, but for a few people, presumably dairy animal farmers, something happened to their genes and they became tolerant, and it was an evolutionary avantage because they had access to all that milk. But because access to farmed milk is relatively recent from an evolutionary standpoint and people can live with lactose intolerance, the trait hasn't spread to all of humanity.