When designing a complex system with multiple business modules and multiple services, correctness becomes a little more binary than that. Your data model tends to proliferate across anything that interacts with it - fixing it may require a complete refactor of all of those modules and all of those services. Throw in a process that is constrained by pesky things like needing to preserve WLB, limited surge capacity, and competing priorities, as well as varying levels of experience on your team, this rewrite can potentially take multiple years and require the maintenance of two parallel systems, and yet it must occur because the data model is foundational for everything else. Continuing to build on the wrong data model will keep boxing you in further until it's too late to ever fix it, with every new feature forcing you to fight with a fundamental flaw of your system. The resulting workarounds and hacks will slowly strangle your business logic until it's too complex to change with any confidence. It's kind of like building a house on top of quicksand.

When faced with such constraints, it's really, really important to get this right from the beginning.