No they aren't the biggest part of the problem, but the difficulty is that we have to solve the whole thing. Carbon emissions need to get to zero and then go negative. That means ideally we need solutions for every contributor, no matter how small.
I agree that cows are an issue that should be looked into, but ultimately cows themselves aren't causing more carbon to be taken from the ground and left in the atmosphere indefinitely (the issue is that methane is a greater contributor to the greenhouse effect over carbon dioxide, per unit carbon), so presumably in this case it's a matter of keeping methane levels below a certain amount rather than reversing carbon emissions.
AFAIK, although methane dissipates more quickly (good in the longer term) it has a more powerful effect in the shorter term.
So the rationale while we are stick deep in the shit is not too think so much of the long term, but just to survive the short term - i.e reduce the methane.
Dissipates more quickly than what? Optimally, the cows would simply release carbon in the form of carbon dioxide rather than methane, since then you simply have a sustainable cycle:
Carbon dioxide is in the air. The carbon dioxide turns into grass. Some of the grass is used as an energy source by the cow, releasing the carbon dioxide back into the air. Some of the grass is used to make up the body mass of the cow, which is then used as an energy source by humans, releasing the carbon dioxide back into the air.
The point is that the carbon dioxide always ends up back where it started, so there's nothing inherently unsustainable. It's equivalent to growing a tree and then burning it (releasing carbon dioxide) to make use of the stored energy captured from the Sun. Humans and cows breathe out the carbon dioxide after the chemical energy is used.
