Mining hardware becomes obsolete very fast, caused by an increase in difficulty. This is an extra reason for miners not to think ahead much.
I used to think Satoshi covered all major parts of Bitcoin, but this may be something he didn't anticipate.
Satoshi is not a god while humans are known to make mistakes including failure to account for something important. I think Satoshi's main omission was that he didn't provide a solution to the problem of having a final say despite the opinion of the majority. Most open-source projects have somebody called BDFL ("benevolent dictator for life") who retains the final say in possible disputes over the future of the project. Satoshi himself could be such a figure but he seems to have lost aspirations for Bitcoin (provided he is still alive in the first place)...
Democracy if applied for real doesn't work, but meritocracy is not without its drawbacks either
I think the biggest misktake he made was to set a hard limit of the block size. He should set a adaptive limit.
"640K ought to be enough for anyone". But how can it be his biggest mistake? This limit would still be set in the code and changed through the code. Otherwise, it would wreak havoc on the Blockchain if, for example, it was a floating value depending, say, on the number of transactions. In the very least, there would be an overhead in determining the actual size of the block if its size were to change arbitrarily...
Most likely an adaptive limit would complicate a lot of other things