Aluminum heatsinks are cheaper than you think. Probably about $5 worth of metal.
Find me those heatsinks for $5 with a MOQ of 2.
Why? Bitmain buys heatsinks by the thousands.
The main impact of the heatsinks on costs is not the manufacturing part but shipping costs.
Bitmain offering an upgrade path is quite brilliant tbh, they incentivise customers to "remove" their old products faster from the network than they otherwise would have. They sell more hardware with less diff impact and even the customers might profit from it, planned obsolescence with a twist.