Generally just against animal abuse and the modern meat industry's intense harm on the enviornment. As a species we've kinda evolved past it being necessary. I don't think meat consumption would be inherently wrong in all instances, obviously we are not herbivores and all of my disdain for the meat industry comes from how it treats animals, wastes product and kills the enviornment. If things were more humane, less wasteful and less damaging to our planet I'd probably have never gone vegan.
As for substitutes you have plenty, from more obvious stuff like Beyond meats to classics like tofu or soy curls that you can use as a substitute in tons of protein-based dishes. Seitan ans mushrooms also work great too, it really depends on the meal. A lot of people will try and tell you that you NEED meat or something, but unless you're on some absolute mindfuck of a vegan diet that doesn't even include protein substitutes or something you'll be fine. In my years of knowing tons and tons of vegans from all walks of life and with varying states of health prior to going vegan, I've never met one who has any long-term issues and most studies seem to show that it really shouldn't be a problem unless you're careless.
The biggest barrier to entry is that its typically more expensive and time consuming to go vegan. I'm sure accessibility to substitutes also varies greatly if you aren't in a major country like the US. Its slowly getting easier overtime here in the states but ultimately I just wish major corporations weren't so shit. Wahoo for the free market or w/e. I could give a more impassioned speech and response to a few particularly silly takes in this thread about veganism/vegetarianism but there are many who get extremely touchy and feel overly judged so I'll lay off. This thread is explosive enough as is and the point isn't to really judge someone for what they do or don't do in a vacuum anyways.
I'll just reiterate that the beauty (and sometimes horror) of humanity is how we've divorced ourselves from anything resembling a normal existence. Doesn't matter what it is or isn't normal to do because we've grown beyond that concept when it comes to how a species is meant to operate within nature by developing such a hyper-industrialized and advanced version of what a normal society could ever be.