What do you think they were doing with adding an entire OCE data center? These things take a lot of time and planning. By their old projections, the new data center would've alleviated a lot of congestion (yes, primarily for JP/NA data centers) and this shouldn't have been a problem.
What they couldn't have predicted is EU player population literally doubling in that time. Nothing about EU's "steady growth since 2.0" could have suggested they need more servers again. Again, as they've gotten increased capacity twice since I've been playing, and for a long while players complained about being spread too thin.
Consider, plans were already underway for the OCE data center, and very likely their roadmap already included more servers elsewhere afterward. Things take time. Upgrades are on the list. But, they've experienced some understandable setbacks. What else were they supposed to do when getting approval for new servers and getting them online can take months to over a year even under normal circumstances? Not exactly something they can say "we need new hardware fast, someone go down to the store and get some." Approval, planning, acquisition, inspections, implementation. It's not a fast process. And they were already in the middle of adding one set, seems pretty unlikely they weren't already in the early stages of adding more.