I think a fair interpretation is if you have both conditions: 1. Is the whole code base a maintenance nightmare? 2. Do we have to change all of it right now?

I think a fair interpretation is if you have both conditions: 1. Is the whole code base a maintenance nightmare? 2. Do we have to change all of it right now? Which occurs, rarely. Most of the time a mechanical transform (e.g. Emacs macro) helps with the annoyances and standards. The rest, a deep refactoring is needed. Frankly, dumping the source and starting from scratch sounds a little like running away from your problems instead of attacking them head-on.