The biggest arguement against a meat diet is quiet simply the current population is unsustainable if the quantities of meat currently consumed in the developed world continue.

Meat production requires roughly 10 times the amount of protein to be fed to an animal for each unit of weight of meat. In other words, you need to grow 10 kilos of plants for every 1 kilo of meat. Soya, wheat and myco-protein based analogues can replace meat in virtally every stew or sauced dish. Some of this (like myco-protein meat substitutes) require things like eggs to make but these can be "free range". Given that it will be necessary to produce some animal proteins, like eggs and milk, there can be animal husbandry techniques that reduce the suffering. This could even involve more advanced techniques like the artificial insemination of female cattle embryos with only a few males for breeding purposes. That would eliminate the source of veal cattle which are usually the unwanted males.

A purely vegan diet is quite difficult for most in the developed world to cope with. If you include a dairy, pulse and grain protein element into every meal, there is little likelihood of any protein problems.

The global warming argument is stonger than you suggest. The principle greenhouse gas from meat production is methane, quite simply all those cows and pigs fart and their dung degrades to methane as well.     The same mass of methane has 10 times the "greenhouse effect" as CO2.