An enzyme to break down milk seems to be such an easier adaptation compared to a whole series of stomach, which are basically a series of fermenters.
Having raised goats the size of their rumens to body is pretty amazing.
Or strong acids and 190°C over a catalyst.
Cellulose is remarkably strong, especially with lignin in the structure too. We're lucky anything can digest it at all.
Which brings me to mycology, probably the best way to extract energy from cellulose for humans. Sterilize, inoculate, keep moist, and harvest a significant percentage of the source material as good carbohydrates and protein.
Which is digestion of course, but it isn't that cows can do it. Our large intestine does the same thing but it isn't anywhere near big enough to get a significant amount of energy from cellulose.