Let’s say those 5 million folks just 1000 API calls a day ( they wont’ actually use ONLY 1000 ).
Correct - 99% of those people will use nowhere near 1000 API calls a day. Each persons subscription only has to cover the costs of their API usage.
A $5/month subscription, at $0.24/1000 api calls, gives each user just under 21,000 API calls per month. Most people aren’t going to be doing anywhere close to that.
This doesn’t even account for fees and possible server costs.
Sync doesn’t need any server costs. It’s not hosting an instance of Lemmy.
Also your maths is off by 1000x lol. The cost would be 1.2mil a day, not 1.2bil a day. You’re multiplying by 1000 for some reason, when the $0.24 already gets you 1000 api calls. So if all 5 million used 1000 calls per day, that’s 5 million x $0.24, which is $1.2mil.