Your reasoning is erroneous. "Humans do not produce lactase, ergo it's false milk is fundamental/crucial". Humans do not produce the enzymes to digest some dietary fiber, ergo it's not fundamental/crucial. The last sentence is false.
Consuming an excessive amount of any type of oligo or polysaccharide will result in basically a laxative effect, GI discomfort etc.
You've got a few options, consume exogenous enzymes (lactaid pills), avoidance, or colonize your GI tract with bacteria that do produce lactase, and feed them a small amount every day (start at 1 gram of milk in your coffee, add 1 gram each day)