That was my first reaction as well, until I did the math.
Turns out watercress has a (my own just-invented term follows...) "specific calcium" of 1200 mg/kg.
Which happens to be about the same as for normal milk (1100-1300 mg/kg).

Let's say a normal person drinks 0.3 l of milk a day. That only "needs" to be replaced by 300 g of watercress, or a a similar amount of a combination of several calcium-rich foods.

No, milk is completely dispensable as there are many alternatives. However, milk is not as bad as some people make it out to be. If you're from Northern Europe (ethnically), you should enjoy your milk.
Excellent quality protein, slow carbs and the fat isn't that bad either, if you're drinking semi-skimmed milk.
That said, there are reports of immunological issues following milk consumption, as well as other problems. Get all available facts, and decide for yourself what's best for you.

I rarely drink plain milk these days, but I do drink buttermilk, and I eat plenty of cottage cheese (all low-fat variants).