As it is, Excel needs to keep around a) the value of a cell, b) its format (ignoring formulae, comments, etc.).
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.
It's not 1987 any more and Excel doesn't use the file format that it used back then. The price of main memory and long-term storage has declined by many orders of magnitude since 1987.
Well, yes, obviously, but do you realise what a headache it is (would be) if the default behaviour of Excel changes? How many people would complain if things that used to work suddenly stop working with a new version?
The problem was that this default behavior was unchangable. Like, let me get into settings and turn this shit off (like they are now doing), they could have done this decades ago.
The value of the cell is what I typed in. How that is displayed is determined at render time or whatever, based on the data type.
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.
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.