I was pondering this, and am shocked that it's not more common. My understanding is that dairy cows average a 3 lactation lifespan due to falling productivity or swing bag. There was a great thread (to which I think the OP was one of the major contributors) that I started about dairy, and she went on about the utility of great udder hygiene avoiding teat loss through mastitis with some really great data.
If we think about the 2 years between "female calf born" and "productive life starts" and then on to "used up dairy cow sent for slaughter", the 20-22 months between birth and first calving are "overhead" which must be amortized over the productive life of the cow, which for simplicity we can break down into milk per lactation and total number of lactations prior to slaughter. Feeding a less productive cow is a cost, but so is feeding a completely unproductive cow while she grows and carries her first calf; none of that is news. We have 3 levers to pull to try to keep productivity high enough to justify more lactations and to allow for more lactations: 1) udder hygiene to keep all of the teats productive 2) genetics for high production, both of which are already HEAVILY optimized by the dairy industry. But #2 tends to work against number of lactations, and selecting for an strong central ligament may be one of the most difficult traits to pick in a cow because selecting for #2 works against it (i.e. a heavier udder can overcome a strong central ligament) and the trait only becomes visible in older cows AFTER the majority of their calves already became veal or were turned into replacement calves. Worse still, the only thing that the farmer can see is how many productive lactations a cow lasted, which observation is beset by a mountain of confounding variables which have nothing do with central ligament strength and longevity. The only real way to grade central ligament health is autopsy (at which point the only way to get a calf out of that cow is to have preemptively harvested eggs before you slaughtered the cow and found out if you just wasted that vet bill) or to invent a procedure with something like an ultrasound to grade the central ligament while the cow is alive (which sounds like a big research project with a vet school). The point being, scientifically selecting for central ligament strength isn't trivial even without considering the difficulty of selecting for other traits at the same time (i.e. you breed "Hurricane", a Jersey with a central ligament built like a suspension bridge cable but with the disposition of a rabid velociraptor).
So let's go back a step and say "how much do these things cost and will the cows keep them on?" I bet some cows hate them, but most get used to them. Nursing calves probably tear them up, and they probably get torn up from a variety of things. But If you're buying them in bulk, I bet it's a $20-30 piece of nylon. If using them adds just one more productive lactation per cow, surely that pays for itself. But what if you leaned into that "crutch", and aggressively bred for higer production, to the extent that you added to milk production from the second lactation onward while the cow bra mitigated the risk of swing bag? Ultimately, your figure of merit is "pounds of fat and protein adjusted milk/total cost adjusted feed consumption". If cow bras improved that number, then I'd buy pink and lacy ones for my cows while the other farmers and ranchers laughed at my fashion sense and envied my extra profits.
Look at that cleavage.
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