I guess I must not be a very good customer because I have yet to experience this

I have seen plenty of places that offer discounts for new clients, but not those that offer them to existing customers which in and of itself always annoyed me.
I think that your system is far more fair-such as the discount I used to receive for paying my bill direct debit instead of by cheque. It cut down on work for the company, in the same way as prompt payment does.