The 200000 vs 10000 (and therefore the 20 vs. 780 man days) come straight from your example. Which is part of the problem: while I know for sure that are a lot of things that should be refactored/changed/improved/cut off it would be very difficult for me to give an estimate of "how much resources we save over the next year" - as a simple example, I could revamp significantly the GUI (God knows how much it needs it)... and in the next six months I get 85% of the requests for changes having to do with business logic, financial batches running at the end of the month and stuff like that, maybe because there are new regulatory hurdles to clear like GDPR or The EU Travel Directive. Then the promised extra resources that would have become "available" fail to materialize, and the whole initiative is considered a failure. Good luck trying this again on the next year.