> around 2015-17 - Swift could have easily dethroned Python.
Why could it?
> it was simple enough - very fast - could plug into the C/C++ ecosystem. Hence all the numeric stuff people were doing in Python powered by C++ libraries could've been done with Swift.
Half a dozen languages fit this description.
> the server ecosystem was starting to come to life, even supported by IBM.
No, not at all. Kitura, Vapor (a fitting name) were just a toys that no serious player ever touched.
After that, and IBM losing interest, Apple did hire a few competent people (including contributors to Netty and Akka) to build the Swift Server Workgroup.
But I don't know why I'd pick Swift on the server when Rust is better in almost every dimension, with a thriving and more community-driven ecosystem.