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.
How much of the code was shared among these services? It sounds like you essentially had mostly the same code running in 140 different configurations with only some translation logic and glue varying between each. I'm not surprised you found this untenable. This is akin to running a microservice per web page.
