Can you name any widely used piece of Software that has not been rewritten at least once? Rewrites: Firefox, IE, Word, Windows, MacOS, ... Many of these have

Can you name any widely used piece of Software that has not been rewritten at least once? Rewrites: Firefox, IE, Word, Windows, MacOS, ... Many of these have been rewritten multiple times and they are all orders of magnitude more complex than some random million-line hunk of perl. Rewriting is hard and may absolutely or just take a long time, but failure to rewrite is pretty much guaranteed to fail.