One club I know of a similar size tried a ticket exchange scheme a few years ago when they were selling out every match. They found that the administration it took them was a massive overhead - ticket office staff were so busy trying to administer the changes and refunds, and field all the calls asking for exchanges, that the service to everyone else really plummeted. Unless they want to invest a large sum (£25,000+) on software to do it all automatically it's a very labour-intensive process - equivalent to 1.5 extra ticket office staff, it was estimated.
As to "pay-on-the-day" - what sort of sensible business would not sell all of their product in advance, but instead would keep some back in the hope that they will sell it at the last moment? That would also require extra staff and cash handling on matchdays, and also give the prospect of people turning up and queueing and not getting tickets - making for unhappy customers.
Which is why I was surprised/disappointed that they failed to even acknowledge a proposal to develop and deliver the whole thing at zero cost. There's a thing in business about "core competencies" and systems development/management isn't and nor should t be a competency of a football club