> Have you actually tried what you are saying ? Because it's a nice idea just completely unrealistic.
I've been in places where it's done. In fact, I've been involved for a long time in a public sector environment with three key systems, one of which is a central claims adjudication system, and two of which are accounting and tracking systems that interface with it, performing similar functions to each other but for two separate programs that share the adjudication system. Those three systems, for historical reasons, have three separate models:
The adjudication is state-owned (that is, the code belongs to the state) and operated, developed almost entirely by a team of contractors with some involvement by state programming staff and a state development lead and also tight integration state-staff IT team that does requirements analysis and acceptance testing and manages most of the interface with state business users.
One of the accounting/tracking systems is, and has been from the beginning, developed and maintained almost entirely in-house by state staff (and almost solely with programming staff.) Because of subsequent organizational consolidation, the state team working on this system overlaps the one working on the prior system.
The other is a vendor-maintained MOTS system, with state IT oversight.
Each has it's strengths and weaknesses, but the MOTS system is consistently the one that is the roadblock to adapting to changing business requirements, and both before and after consolidation the all-internal one was the one with the quickest best-case requirementd to delivery speed, and by far the least expensive for the value delivered.