Sorry, I just didn't want to give people the impression that it's more difficult than other languages. To upgrade you'd just have to remove the original install directory and untar the new release.
Also, upgrading will be less common than Rust since almost everyone uses the latest release/toolchain. There's not really a reason to use the daily builds unless you're contributing to the Swift project.
For comparison, for Go, it's "sudo snap refresh go". For Rust it's "rustup update stable". I mean, how hard would it be to properly package this stuff, and why should tens of thousands of users deal with all this manual downloading and unpacking? Assuming, of course, that Swift folks don't deliberately want to make the language unpolular, like Haskell.
I agree they should put the effort into having an easy "apt-get" solution. On the other hand, your original question was whether there is a standard way of installing Swift on Ubuntu. The answer is a very clear YES: download the latest tarball of the binaries from the Download page and unzip.
And that's fine: ship an official snap or use the Rust solution. Not doing this very directly impacts adoption. Most people won't even try to set it up.
I'm not familiar with Snap, but I did find https://snapcraft.io/swift
Also, upgrading will be less common than Rust since almost everyone uses the latest release/toolchain. There's not really a reason to use the daily builds unless you're contributing to the Swift project.