Both cows, horses and even to a limited extent humans can digest fiber. Cows digest fiber in the rumen where it actually turns mostly into organic acids which the cow can oxidize while the anaerobic rumen bacteria cannot. Interestingly the same thing happens in the large intestine in other mammals. For humans the large intestine is quite small and food moves through there too quickly for much fiber to be properly digested. However the easiest digestible fiber, soluble fiber, actually mostly breaks down even in a human’s large intestine and yields us approximately 2 calories per gram of soluble fiber. For insoluble fiber this amount is extremely low since there is not enough fermentation taking place for it to be completely broken down. However for mammals with a much larger large intestine where food passes much slower, even the harder to digest fibers can be utilized to a large degree.

Horses belong to this category and are called hindgut fermenters. Other examples may surprise you like gorillas and orangutans who have incredibly huge large intestines. That’s why those apes can eat leaves all day and is an explanation why their stomachs are huge without them being filled with fat, it’s all intestines.

However a weakness with hindgut fermentation is that the large intestine can only extract solubles from the microbial mass which leaves out a lot of nutrients. A cow can extract those same organic acids from the fermentation but since the rumen is first in their digestive system the whole microbial mass enters their “ordinary” digestive system which means that they can digest the actual bacteria as well, meaning they manage to extract a bunch of extra microbial proteins that hindgut fermenters may miss. The benefit to hindgut fermentation is however that the first shot at digesting the food is given to the animal itself. A horse can digest starch just as well as a human could but a cow suffers considerable losses in starch digestion since the bacteria gets first gibs, turning the starch to organic acids instead of getting broken down into simple sugars directly, which is more efficient. So in short a cow and horse can both digest fiber. However their digestive systems have significant tradeoffs and one is not necessarily better than the other.

Considering that’s a Holstein breed cow and therefore a milking cow, the nutritional demands are entirely different. Getting a cow to produce 40 liters of milk a day is no easy task and requires grass of the highest quality, combined with a generous dose of concentrate feed with grains and legumes/presscake. If a normal hobby horse was fed a diet like this they would turn obese almost instantly. In fact hobby horses usually require as poor quality feed as possible because it turns out that being ridden at walking speed for an hour 1-2 times a week is a very low amount of exercise for a horse. You have to intentionally grow as rough and low quality grass as possible for the horses not to get obese. That’s why oats are no longer given to horses. A race horse or a working horse that’s active for several hours a day can however be given oats or other concentrated feed and may be able to handle, or at least come close to handling, a dairy cow type diet. However these types of hard working horses are rare nowadays.

TLDR dairy cows and horses generally do NOT eat the same diet.

In the US the native herbivore with the “cow-niche” is the American bison. If we would restore ecosystems and replace captive grazers with wild grazers, increasing the wild bison population is the answer and much preferable to having wild cows (who don’t even exist in the first place, the wild version is extinct as mentioned). Of course bison is not an answer to what to do with the cows that already exist in the US of course.

However if a decision was made to ban all animal agriculture I would be a strong opponent of not rewilding any cows. They are not native and they are not even fit for living in the wild anymore. Just take a Holstein milking cow for example. What use does producing 40liter of milk per day have in the wild? None! Calves can’t drink even close to that amount. The lactating moms would get mastitis. They are not even fit to only make milk for just their calves anymore. Let the domestic cows die out in that case.

Well no shit. That applies to most animals we humans care for, even the ones who we don’t typically eat. Try throwing a hairless cat or a pug out into the wild. They can’t manage without us no more.

Interestingly enough you don’t have to be so specific as Black Angus. All cows are totally extinct in the wild. They derive from the Eurasian auroch which went extinct in most places of its original range over 3000 years ago. The absolute last one died in 1627 in Poland, but even that one was probably not pure auroch. If everyone went vegan we would probably still keep a few cows around in zoos but we would have no where near the amount we have today. If we wanted to reintroduce something similar we would have to rely on reintroducing european buffalos, which are another species but still native to Europe.