It seems like a waste of time to change defaulting from C: to A:. It seems to me that the only point of doing this is to attempt to break programs built with dodgy programming practices, which isn't really a direct benefit to the end user ("great, my discontinued app from a now non-existent developer won't work on Windows Whatever"), as programmers who engage in dodgy coding practices will find a hundred other ways to do their job badly.
If MS are going to bother changing anything, either change to a UNIX style method of referring to available storage, or something else entirely like AmigaOS where a storage device is referred to by volume labels (the default in AmigaOS has been to refer to the system partition as "Workbench:" and the data partition as "Work:", or by drive codes like "DH0:" if that turns you on). That would have the benefit of dispensing with the limited number of drive letters to the end user, but then, how many users have this problem?
However, considering that one of Windows's advantages for a long time has been backwards compatibility, I'd say leave the default OS installation drive letter as it is.