A thing that bothers me when people use the "veganism is expensive" argument is that they're useing it against veganism itself and not against the system we live in, capitalism and such related things that are actually to blame. "The only store that sells vegan milk is miles away and it costs like my kidney!" then try to make change! Make more stores sell vegan milk and for more accessible prices! Realise who is really at fault!
I think that it’s helpful to remind people that animal products are only relatively affordable at the point of purchase because they’re heavily subsidised by the taxpayer. The price you see at the checkout isn’t the actual price, you’ve paid the rest of the bill through your taxes. Even then, the days of cheap meats and eggs are mostly gone, they’re now some of the most expensive items in people’s baskets.
That doesn’t make much of a difference when you don’t have any money of course, but the idea that being vegan is inherently more expensive is just not true anymore, if it ever was. Even the lowest quality meat is far more expensive than a wisely chosen plant protein, like TVP, lentils, chickpeas or beans. There is a reason why food outreach organisations almost always use mostly vegetarian food - because it is cheaper and you can feed far more people with it.
Food deserts are a whole different issue, and it’s systematic as you’ve pointed out. The good news though is that the more people who choose plant based food, the more alternatives there are. That means an easier time for vegans yes, but also vegetarians, lactose intolerant people and people with religious dietary requirements. If there is demand and competition, plant based options become more accessible overall. We are already seeing this in countries like mine (The UK), where supermarket own brand milks and meats are already very affordable.
