There are countless stories of large scale failures involving non-consultants as well as very well paid consultants.
Software is hard and most developers aren't very good. As the complexity of software increases, the number of developers and teams that can manage it becomes vanishingly small; to a point where no amount of money can help.
The best chance for a project like this to succeed is to break it down and migrate over the course of years (which might not always be possible).