I see a potential road-block for Swift's becoming really popular as a server-side language: The default ecosystem for Swift/Vapor development seems to be macOS + Xcode, and while an Ubuntu version is available, it 1) isn't available for the newest Ubuntu version yet, and 2) there's no Xcode on Ubuntu, so you'll miss out of the things that Xcode can give you that a command-line Swift interpreter can't. Even Vapor's own hello world example relies on Xcode. And if you're on Windows, your only option seems to be running an (outdated) version of Ubuntu in a virtual machine.
Point is, while Swift and Vapor are great, Swift may not reach its full potential unless developers are less tied to a specific OS and editor/IDE. What do you think?
(Btw., when asking a question like the above, some people will say, "just use the best tool for the job". This is a common opinion, but in reality the popularity of a language and framework means a huge deal when it comes to getting help, looking up answers, hiring programmers, hosting your project easily, etc.).