Answer
Stable, well-documented trading partner relationships can often stay in-house without much risk.
The realistic constraint for many IBM i shops is not budget but bench strength. EDI mapping expertise tends to sit with one or two long-tenured people, and when that knowledge lives in someone's head rather than in documented, versioned maps, the in-house option becomes riskier the moment that person is unavailable, whether from vacation, illness, or retirement. Buyers should honestly assess how much of their EDI operation depends on institutional memory before assuming staying in-house is the lower-risk path.
Managed partners earn their cost when partner onboarding is frequent enough that a dedicated team's efficiency offsets the ongoing service fee, and when the business wants a contractual response time for EDI problems instead of depending on internal availability. The middle path many teams land on is hybrid: keeping stable, well-documented core partners in-house while routing new or high-maintenance partner relationships to a managed service, which limits the blast radius of any single point of failure on either side.