This is true, but a tiny proportion of people buy merchandise compared to music. You will always find the t-shirt stand easily accessible at a gig for the artist despite the thousands of people at a venue, so believing that t-shirt sales will someone keep the artists profitable is likely wrong.
I don't have the personal experience from gigs with "thousands of people", as the only ones I've been to were not that huge, and at those the merch stands were always swarmed. If you are playing for an audience that big you are probably already good on ticket sales alone (which also tend to become more expensive once a certain audience threshold is crossed).
I also wasn't just guessing/assuming that, but relaying what I've been told by friends who are musicians (see also my sibling comment).
