My guess is that the system-level changes they made (elemental affinity, dynamic affinity changing, chaining changes, new XP tables and splitting calculations, leveling system changes incl. level down, etc.) were a lot more complicated to get working in FFXIV's codebase than people would believe. I highly doubt it's like they had an entire team of dozens of developers working on nothing but this nonstop for years, but probably the amount of staff they have who can make those kinds of system level changes to the server code are very few, and it probably had all sorts of bugs and complications with many iterations just to get to this point.

(This is also why I'm guessing that making the types of changes people are requesting now (adding things on top to make it more interesting) will probably be much smaller in comparison, having built this foundation.)

Of course none of this changes the "that's it?" feeling you're alluding to at all, and that's still a pretty major misfire. But I suspect this will be one of those cases where the work involved will mean a lot more to the people inside the team (who know what was really involved) than to anyone on the outside (who, rightfully, only judge the final result).