I'd also like to add that there are other, non-financial parameters adding to the value of AFT such as strong implementation in society and becoming a household verb. It's like "Googling" something on the internet, if you want to buy something now, you just afterpay it.
In addition, Afterpay can always increase their merchant fees. A small increase of 0.5 - 2% at 100 million potential customers will increase EBITDA dramatically.