Meat alternatives are perfectly fine. And tons of people do perfectly fine with zero meat at all and thrive just as much as people who eat meat daily. I have no qualms with eating meat since I do but let’s not kids ourselves and say it is a necessity.
The big problem with beef is the amount of land and resources it takes. It takes a fuck ton of water and feed to get a pound of beef. The added carbon from beef is largely due to transportation of the feed, electricity, and also transportation of it on its way to the store. If that were all green sources, cattle would basically be carbon neutral. We are a long way from that though. And even if the energy sources for those were green, the other resources they eat up leads to massive destruction of environments.
Animals can certainly play a part in sustainable farming but the amount we currently have is absurd and is nowhere near sustainable. Just killing the subsidies alone would bring it significantly closer to sustainable. If the US stopped providing subsidies for the cattle industry, beef would be $35/lb.