All of the drives in the RAID were the same age, so it didn't make a lot of sense to replace only three of them.

That reduces the price per drive to a mere $1,000! What a steal.

More importantly, why was their database running on a physical server?

Edit: why the downvote? I don’t care how massive your database is, it doesn’t need to be running directly on a physical server.

They've update with a picture of the replacements, vnand ssd 860 pros.

Even the 4TB variant appears to be around $1k pre-tax at most, though.