That was technically their business model, but they gave you one year free and also extended your 'subscription' indefinitely if you decided not to pay after that - basically the WinRAR of messaging apps.
On iOS the app used to be $0.99 (one-off), which probably made them a couple millions. In 2013 they moved to a subscription model [1] but this was never implemented; I think nobody was ever charged.
If you live in a wealthy country, you probably "lost" more money writing that comment than what you paid for WhatsApp, even paying twice.
Completely unrelated from anything. People don't come on HN to make or save money.
I paid for it too. I like to think that "idiots" like us prove that there is demand for a paid ad-and-tracking-free messaging service, so maybe our efforts weren't in vain.
Until the facebook acquisition it cost about $1/year. IIRC it was even profitable.