if we all became vegan what would happen to the billions of cows currently being bred for slaughter?

there would be a short transition period where two things would happen:
(1) no new calves born for slaughter
(2) the cows which had been born for slaughter will die out naturally over time (10-15 years)

an ideal relationship where we protect the cow and the cow lovingly returns milk and other products

...currently not the case with most of the milk and milk-products in circulation even in India, land of the Vedic tradition of bovine respect! I accept your point where there is mutual respect, but this is not to be found in factory farms where the cow is raped (artificial insemination), traumatised (calf taken away at birth) and tortured (miserable living conditions, pumped full of steroids etc, milked to an early death at around 5 years instead of 15).

Having said the above as if I am not in agreement with where you are coming from, your post is excellent and the three reasons you give are ones that I would too 🔆 . Resteemed!