Re: I must be missing something...
> An option in Excel settings. That's all what is needed.
No one would use that option. If there is no guessing then when opening any text file it would have to assume all columns are text. That would make any genuine numeric and date data unusable without the pain of excels data type conversion functions.
There are 2 problems with the current behaviour
First it guesses on a cell by cell basis, not a column basis. So if you have some values that might be a date they get converted but the rest of the column looks OK.
Second there is no visibility of what's been converted so when the user sees the first screen full and it looks alright they assume it's all good.
Even a notification on opening a non-native file that it is being interpreted/converted would be an improvement - I'm sure I'm not the only person in financial services that's had to break out the luhn algorithm to regenerate the last digit of credit card numbers after a csv file has been opened and saved in Excel.