IT is very simple and easy to implement.
You make a tender, evaluate their different bids and then select a supplier based on a set of criteria. The supplier then provide a turn-key solution and the only thing you have to do it accept the final delivery. How hard can it be?
Well, the harsh reality is that life is a little bit more complicated than this. Unless it is a totally new company, with no legacy IT or impact on the existing business, the turn-key solution is more rare than a unicorn.
There are very few IT projects that doesn't involve business and gives a business benefit.
The only exception is when exchanging old hardware with new one and where there is no new functionality. Consolidation of infrastructure may be in this area, but then we have the legacy angle to take care of. The daily operations could be impacted in several ways and there could be an impact on development projects.
Why does the turn-key delivery have impact on operations? It is as simple as someone need to know how to operate the solution after go-live.
Instead of a turn-key solution you could see it as different layers, where you have to define different responsibilities. You could hire a system integrator to take responsibilities for technical infrastructure, applications and integration, e.g. system implementation, but the program will have to manage dependencies to legacy applications in one way or another.
The business value will always reside in the unit in the organisation with P&L, or higher. Period.
Unless it's a very small change, there will be an impact on the daily work for the people in the business organisation. This transformation have to be handled by somebody, and this is often not the core business for a system integrator.
As a program manager for a medium to large size program, you have the responsibility, on behalf of stakeholders, to assure all parts that are needed for the business value and business transformation are in place. As the work always is done by separate parties, with different set of priorities, you have to do the coordination between all of them.
Another impact of this approach is that fixed price sub-projects only works if there are no delays due to dependencies between streams. The logical solution is therefore to limit the time and scope for each assignment, without going to the extreme, i.e. a more agile than waterfall approach.
The only way to get rid of the responsibilites you have as a company during a change is to sell your company before the transition. Sometimes this may be the best idea.