Peter Frimberly wrote:

This in mind, I expect they'll drop the deposit business soon, and
just call it a "card setup fee". Anyone that wants to keep paying mega
inflated rates to get paper tickets rather than spend £3 on this "card
setup fee" will be perfectly able to do so.

The trouble is that the touts will happily take on that role instead.
Having a deposit rather than a fee tends to discourage that problem.

Neil