I still use FTP to live update production sites. I do this for my personal blog, and on my projects that don't get much traffic.
It really only becomes a sin when you have a lot of visitors who are relying on your site. For my larger sites, I have a build process. It's all a mater of scale and how much risk is acceptable for your website.
I think the audience of devs uploading a single PHP file have moved over to CMS's. Wordpress, Joomla & Drupal have very much taken over that, which in a way can be a good thing. They can be lightweight, easy enough to use and can grow as big as you want. The other big reason is plugins/component/libraries too.
There's absolutely nothing wrong with it in principle.
It's only that as the web grew up, and websites became less a bunch of static files and increasingly an application, or more commonly a pile of application-like messes run by many different people at the same time, involving a bunch of dependencies between files, whether organized code or haphazard things piled on things piled on things, doing staged releases comes to be the more sensible option.
At a certain level of complexity, FTP can become a liability and cost you business.
There appears to be a vocal group which suggests their way of doing it is best, even if it involves trudging through configuration documents that remind one of engineering schematics.
Not knocking down the many brilliant developers who work this way, but it seems the simple approach sometimes is the best approach.
It really only becomes a sin when you have a lot of visitors who are relying on your site. For my larger sites, I have a build process. It's all a mater of scale and how much risk is acceptable for your website.