Most of these libraries are open source. One we had the most friction updating was was our library that wraps common logic for dealing with user generated events - https://github.com/segmentio/facade (this is the example that was referenced in the blog post).
At the time we did the split, we had <30 destinations and 3-4 engineers working on this system. At the time we decided to merge, we had > 140 destinations with ~8 engineers, of which roughly 3 full-time engineers spending most of their time just keeping the system alive.