But they aren't going to replace milk that we use in, say, the production of butter, cheese, casein, or lactose products.
I'd be curious to see what a pie chart of the end products from our milk production looks like.
There are excellent butter, yogurt, and cheese products made without dairy. The cheese is made from nuts, not plant-based milks.
Or maybe you are just talking about "soy cheese"? I agree that pure tofu is pretty bland, but with various seasonings (I'm partial to Chili oil), it can be quite good.
It's not as obvious as, say, olive oil vs. canola oil, but it is distinguishable. I can imagine giving up butter in favor of margarine... but then I also doubt the half pound I go through per year has a major impact in the big picture.
As for cheese... I do eat tofu sometimes in place of meats, but don't see it as a good substitute for cheese, except for perhaps a very narrow selection of cheese varieties (perhaps paneer).
I've yet to find a soy substitute for a cheddar or brie. What I have had is soy ricotta, which is about 80% of the way to matching the real thing in terms of texture and about 50% there in terms of flavor. I have no doubt it will be done eventually, but we're still a _very_ long way from replacing a cheese lover's dairy consumption with alternatives.
The point I was trying to make is that replacing the milk sold largely for drinking probably doesn't do much of a dent in the industry demand for dairy.
To make a real impact you'd probably have to replace those uses (or get people to radically change the types of food they eat).