Years ago I had my own clock of sheep (between 30 to 50 ewes). Yes, we need lambs for meat, but all of my animals had the very highest standard of welfare.
When it was time for "the final journey" we booked the first slot at the local abbatoir, borrowed a trailer, loaded the animals ourselves and took them.
They had the minimum amount of stress possible, and the meat tasted better for it. (Stress produces adrenalin, which hardens the muscles).
When I mention "mess" a couple of things that come to mind are the processing and waste products involved in producing alternative milks (soya, almond etc), plus the amount of land that would have to be given over to plant production rather than grass or pasture.
Large arable farms are not interested in trees or hedgerows, and the removal of these results in more water run off (my journey to work these last few weeks has seen so much more standing water on the lanes and roads).
If wool or leather isn't worn, what replaces it, how is it produced? Artificial materials aren't especially environmentally friendly, flax, linen and bamboo need large areas of land, and the processing of those bring problems too.
I think that an (albeit) idealistic way is to go back a couple of hundred years, where meat and plant based crops were produced and sold locally, no transporting animals God knows how many miles, with the benefits of (some) modern practices i.e. veterinary medicines for animal welfare, crops that are reliable and disease resistant without resorting to GM.