* NEVER modify the text I type into a cell.
* Parse, interpret, format the cell according to the "data type" that's detected or chosen by the user
* In the grid, display the data according to the interpretation above
* In the formula textbox, show me exactly what I typed in, unmodified.
One extremely aggrevating thing it does repeatedly is strip leading 0s off phone numbers (local phone numbers here follow thw format 0XX-XXX-XXXX), we constanly need to work around it by remembering to add a dash in the middle.
In your proposal, Excel would need to keep around a) the value of a cell, b) its format, and c) whatever you originally typed in, with c) being potentially more than a)+b), without very much benefit. And remember, Excel was released in 1987 (years before Windows 3.1) on machines that had less storage than they have today. I'd say the design decision made back then was the right one.
This is already kind of the case. If you change the date rendering format, the "real" date text in the cell stays the same. The only issue is that Excel decides what an acceptable "real" value is and modifies my input to match that.