While a beta test would've been ideal given no time constraints or dependencies, I think it would've done more harm than good in this case.
A beta test would've postponed the update by at least a month, even if we received almost no feedback (and only a tiny proportion of players participate in betas, so gives us the tiniest sliver of player eyes (ouch, that's a painful metaphor!) compared to the issues we were able to find and correct yesterday).
We needed to align the update with the last server-side engine release scheduled for this year, so we wouldn't've released 'til 2021 if we beta tested.
Meanwhile, updates in (and nearing) development need to add achievements, which the old system could no longer sustain.
We've had to be incredibly frugal in releasing achievements this year until we overcome this obstacle.
No more achievements means no quests, no Boss Slayer, etc. So postponing engine-side achievement caching would slow our release cadence, and players have been pushing hard for gameplay-additive updates to be more frequent.
Considering the volume of data we had to recreate, very few issues with the achievement data have slipped through. And due to the elegance of the new system, we already have fixes prepared to address them in the next release.
There's a larger question there of beta testing as a standard element of the development cycle. We likely wouldn't be able to update weekly if that were the case. Updates would likely either be released with more known issues or would be postponed to go back under the knife much more often. Perfectionism can be paralysing. Would it be worth it?
The feedback loop would be helpful to make releases more stable, but there's always more you could do to make an update better, and always something else that's higher priority, so at what point do you draw the line? Fortunately that's not my decision to make.