The comment system bundled with Wordpress is not enough for "serious blogging", you'll need plugins to manage spam, allow users to use social accounts, etc. You'll also need plugins for caching. Oh, and you'll need plugins for SEO as well, because the core Wordpress doesn't handle the bare minimum of SEO and isn't flexible enough to allow you to do it yourself. And each plugin expands the attack surface to a system that's known for the persistent attacks, so you have to be constantly looking for updates and keeping everything update. Oh, but not everything, because there are breaking changes. And there are versions that introduce bugs or regressions.

But yeah, a bunch of html pages is over-engineering.

PS: you don't need to use Disqus if you don't want to, there are a myriad of free open-source alternatives you can self-host that will take a fraction of the maintenance work of self-hosting a Wordpress instance.

You don't need to manage spam if you're willing to moderate comments yourself. But a plugin to integrate with Akismet can be installed in a single click. To get the same thing with an SSG, you'll have to either build your own comment system and integrate with an API, or use a third party commenting service.

Social accounts aren't necessary either. Users can simply not login, or use their email address. If you still want social accounts, it's a one click plugin install. With SSGs, you have to bake it yourself or buy a comment system that includes social auth.

SEO is a big deal, and Wordpress doesn't allow it by default. However, you can install a minimalistic SEO plugin like "The SEO Framework". If you want more advanced SEO, you can install a plugin like Yoast. With an SSG, you have to build your own theme that will allow SEO using frontmatter from your HTML pages, or use an existing theme that has that functionality.

Adding plugins introduce additional attack surfaces, but if you're building a full-fledged SSG to rival wordpress, you're connecting to various APIs, and writing your own code that could easily introduce security bugs. Wordpress plugins can auto update, but SSGs require you to fix bugs on your own time.

Wordpress version changes can break your site, but you can easily backup and restore your content for easier migrations these days. SSGs can also break your site if you want to keep the generator up to date. For example, if you use Jekyll 2, you'll have to spend some time refactoring your project when upgrading to version 4. Now, if you have integrations that you built yourself, this could take considerable effort.

I don't personally see how an SSG can compare to Wordpress. Sure, if you enjoy writing Markdown files in Vim then go ahead, but for people who mainly want to write content and provide a service that visitors can interact with, Wordpress wins all the time.