Veganism is a positive lifestyle choice. The point is - don't judge others for drawing their line in a different palce. They may well be living a "better" life than you, but in different ways.
The 1st thing is that 'vegan' isnt a dietary choice; secondly the vast majority of vegans I know make other choices against the things you stated such as pollution etc..
I would also i would say in many areas of human existance the lines of bad/good/better life choices arent clear (eg fossil v nuclear power many facets to the argument). that is just a simple example. it makes the drawing of an arbitary line more difficult.
However, we know that mistreating/abusing animals and killing is wrong, we have the ability to easily reduce this in the western world, yet people make the choice not to, mostly in favour of greed. I dont understand how people can defend that position?