Nobody doubts one could build something similar to Aurora given enough budget, time, and skills.

But that's not replicating the experience of Aurora. The experience of Aurora is I can have all of that, in like 30 lines of terraform and a few minutes. And then I don't need to worry about managing the zpools, I don't need to ensure the heartbeats are working fine, I don't need to worry about hardware failures (to a large extent), I don't need to drive to multiple different physical locations to set up the hardware, I don't need to worry about handling patching, etc.

You might replicate the features, but you're not replicating the experience.