I think you've stated this in reverse.

API limits are infinite but you'd blow through $20 of usage in a maybe 1 hours or less of intense Opus use.

The subscription at $20/mo (or $200) allows for vastly more queries than $20 would buy you via API but you are constrained by hourly/weekly limits.

The $20/mo sub user will take a lot longer to complete a high token count task (due to start/stop) BUT they will cap their costs.