You can actually notify the Janitor when there are new commits (and possibly tags indicating releases) in an upstream repository. The URL to target in the webhook is simply "https://janitor.debian.net/", and supported webhooks are those from GitLab, GitHub, Launchpad and Gitea.
Note that vcswatch doesn't actually scan upstream repositories, just Debian packaging repositories.
I don't disagree - I'm not suggesting anywhere that this be used to directly upload to unstable. There's definitely a class of users who would like the balance between freshness and quality to be closer to freshness for some packages.
There's a big warning on https://janitor.debian.net/fresh about the packages not having had human review, and the last paragraph of the blog post mentions this as well.
That said, I do believe strongly that we can let automation take care of more of the (boring bits) of the packaging process even for unstable, and leave the interesting bits that automation can't help with to the humans.
The process is indeed complex, but there are efforts to standardize on a single simple way to do packaging. Unfortunately with an archive of 30k source packages, that is a slow process.
A great resource is https://trends.debian.net/, which tracks some of the different ways of doing packaging, as well as the progress on convergence.
With the new style debhelper that we're converging on and a straightforward package, creating a new package should not have to involve a lot of typing - although it's still split across multiple files.
This is fixing issues in the packaging, not the upstream part of the package. I have plans to also have it fix issues in upstreams, but as you say - those will go into the upstream repositories.
It processes the main Debian archive, not PPAs. The bot makes changes to the Git repository with the packaging - developers would get a pull request with the changes after doing verification, and they can just click "Merge" on the GitLab UI. That seems pretty similar to what you're saying would happen in arch.
Note that vcswatch doesn't actually scan upstream repositories, just Debian packaging repositories.