But there's literally decades of use cases and experience contained in that code. That's one major issue with people thinking about rewriting software, they vastly underestimate the amount of things in there.
I mean on the surface SAP is just a database with a UI and some business rules - 99% of software is - but that in itself is so often underestimated.
(Source: anecdotal, I underestimated 'just' a configuration interface, except that it had hundreds of models and thousands of fields. In hindsight I should've looked for an off-the-shelf data management thing where all I would have had to do was configure the fields and validation rules and some rough layout. Instead I tried to build it in naive Go, sqlite, a REST API and a React/bootstrap front-end. It's a great solution IMO, but not if you're a solo developer working on a domain that big).