Cattle don't prevent monocultural ecosystems, they are monocultural ecosystems. Prime example - the Amazon Rainforest - a fucking cathedral of biodiversity, is chopped down to make way for cattle ranches by the hundreds or even thousands of hectares each week. There's no ecological joy in livestock.
As for cattle 'making grass useful' - the grass is often only there because the land was cleared for livestock.
And local plants only growing certain times of the year - more bollocks. A field of wheat will feed hundreds of people. A field of cows will feed a tenth of that. There's no comparison. Just because a crop field may be unusable for a few months of the year, that doesn't mean its somehow less economical than livestock.
I say all this as someone who eats meat very regularly, so I don't have some kind of vegan axe to grind. Your points are bollocks!