[mmcdermott]

Figuring out the requirements is a research project unto itself. The churn of people over the years means there is no single source of truth for what the system does or why. I wrote an answer about this at https://softwareengineering.stackexchange.com/questions/4252...

You generally want to improve on what you have in some way (or else why do it?), but the line between the warts and requirements is blurry.

Incremental replacement is often a hard sell on the relationship side of things because the customer is seeing a reduction in functionality until everything is done. You can mitigate it and work around it, but it tends to excite waterfall urges which drives up risk.

You also have the fact that the organization keeps changing, so your target is also changing.

There is also an opportunity cost to the whole venture. All that time you spent rebuilding what you had could have been put into new ventures or back into core business.

If you look into modernization type efforts, you see a lot of failure and a lot of loss. That isn't to say it's impossible or that it isn't ever the right answer, but it definitely isn't safe or easy.