In October 2017, annualised sales were $1.5 billion with quarterly sales of $367 million, 11.5 times higher than what SPT just achieved. To achieve that in 1.5 years from now, compound average QoQ growth will need to be 50%, which is huge. APT needed 8,600 merchants for this.
You're making money not because your predictions in the fundamentals have been proven correct, but because more and more others have bought in with the same hypothesis/ in for a quick buck. The amount of risk in achieving the same as Afterpay has hardly changed since it IPO'd - there's been no update on merchant count, underlying sales, or customer count. All that's changed is the share price (5.5x), greatly decreasing the risk-reward trade-off.