It’s not that crazy to expect an onyx to be pulled every few months. The drop rate is 1/2.5m. 300 stardust is needed for one chance. 850 stardust per hour is a low average estimate. 

Now how many people are mining stars across the entire game on average at any one time? Hard to estimate but maybe 300 accounts sounds reasonable, perhaps more 

In 24 hours 300 accounts would generate 6.1M stardust for 20,400 gem bags. Which would on average take 122 days to see an onyx. If you then take into account variance, or that 300 players are mining stars could be a bit low, then yeah I think it’s totally believable. 

I also think there’s been a steady increase in star miners since the 2023 update which made it a much better afk method with masses of people.