One of the huge pitfalls in the writing from scratch situation is that often times the original application didn't have adequate requirements. So the team sets

One of the huge pitfalls in the writing from scratch situation is that often times the original application didn't have adequate requirements. So the team sets about building the new version without a comprehensive and detailed understanding of what the old version did. That alone can lead to huge timeline and cost overruns. And if your organization didn't learn from its mistakes the first time around, it'll make many of the same mistakes the second time around, except this time they'll be concentrated in one monster release rather than a number of smaller releases. Especially if the original developers have left.