With 100 mildly competent developers on staff (although not with SAP developers) a company can develop in house everything they need to run a business in a period of time that is shorter than what is needed for a typical "SAP adjustment".
Specially with the tools that exist in open source. Amazing databases, data modeling systems, libraries to integrate into literally everything.
It think the bigger problem might actually be to define the process you want to implement. What are the specifications, what of your existing system is actually needed and what isn't. Rather then the actual implementation of those processes in some digital process.
I have a friend who work himself up from worker in a warehouse to team leader and now SAP something something manager. And from the story he tells me I am mostly constantly confused. The whole environment feels incredibly foreign.
Build a inventory planning and tracking system for a company that operates in every country in the world and has to issue and receive invoices, customs documents, waybills etc in hundreds of formats, currencies, languages and accounting standards.
And you are right that the biggest problem is definining processes and data types. Whatever waybill system a 100-head HN Rust and JS crack commando comes up with, you can rest assured that there's some authority somewhere that wants an item displayed differently, so you either patch this and many other edge cases until your codes becomes an unworkable spaghetti, or you go back to the drawing board countless times and ship nothing, or you outsource it to SAP and the likes who have spent decades coming up with a working solution (even if it's not that elegant).
You really don't just need 100 programs but you need 30 people defining the processes and 200 configuration experts.
This really seems like something the world would profit from creating a huge amount of open libraries for all this stuff. There are so many old apis in all these old systems. I work in banking and when switching company and then you re-implement the same shitty format again.
You would obviously need some people who have done ERP systems experience, not just 100 random developers.
I think, at least I have heard, that Tesla does its own internal system, would be interesting to see how many people they have working on that if its true.
Posters in this thread are so clueless it hurts to read.
But also, SAP has a lot of accounting and legal process knowledge baked in that is reusable even if you customize everything about it. That's what "mildly competent" developers are missing.
Essentially, they used SAP as a framework.