Another comment suggested you try to appeal to empathy. It might work, it might not.

If it doesn't you can instead appeal to the bottom line. Tell 'em that they might get targeted by an accessibility troll: https://www.wrightlawgroup.com/blog/attack-of-the-ada-trolls

As far as trolls go, I consider these fairly benign, as their behavior actually promotes something good; we got hit with such a lawsuit, and all of a sudden a11y became a big thing as our CEO didn't want to give a dime to those 'greedy assholes'.

This is exactly what happens. Especially if your voice is in the minority Or you’re the only one. You’re seen as a trouble maker. We all know what happens to trouble makers when margins are on the line.

I just stab them in the eyes and then say "now you'll care about blind people," then calmly walk away. Works every time. /s

In all seriousness, I ask something similar to the above: "Would you feel the same way if you got in an accident that left you blind for the rest of your life?" Sometimes this moves a few people to your side. I don't want to be the only one, but I do care about accessibility. I care a lot, for personal reasons. But like you said, you don't want to be the minority trying to support minorities.

I think there are some laws which require a certain degree of accessibility for at least government websites. And if your big company deals with government, they'd better think about that.