This is also the core motivation behind all of the schemes to force a single coding style on all developers at the cost of performance and usually with an extremely complicated design for seemingly no reason. Usually, these turn out to be unreadable messes as well a generation later once everybody has moved onto the latest and "greatest" coding style trend.
Instead of doing rewrites, you can just refactor code. That means, among other things, deleting obsolete code for platforms you no longer support and, in the case of C/C++, converting macros to functions. You can also document the codebase (with comments not commit messages) so that developers understand what the code does and why it is there. But a full rewrite is usually a bad idea that will just introduce new bugs and remove existing functionality.
Refactor vs rewrite is basically the software development equivalent of reform vs revolution in political science and everybody with even a minimal knowledge of history knows that revolutions usually end badly with an even worse regime taking power. Likewise, the failure to accept the need for reform (i.e. refusing to allow refactoring) inevitably leads to revolution.