the main assumption is that everytime he debugged his client and updated it, it created a new wallets/files and be didnt back up the previous
take this quote that shows early versions needed backing up entire folders.
imagine backing up a folder every time you edited a line of code
4. What files need to be backed up for not losing my "money"? Only the wallet.dat or the whole Bitcoin AppData directory ?
Version 0.1.5: backup the whole %appdata%\Bitcoin directory.
Version 0.2: you can backup just wallet.dat.
satoshi's 'stash' were 50 coins per address which if 1mill coins was accurate would be 20,000 addresses, in numerous folders, if he did backup
remember from january 2009-mid 2010 bitcoin had no value in the real world. it was just a experiment back then so there was no value in keeping the private keys. assumptions were that after a few months of bug fixing everything would be reset and born again. but too many joind and it instead just continued to live and it wasnt until 2010 before h made it simple to backup