One of the things that Gemini critics miss, and that Gemini proponents don't do a good job of describing, is that Gemini represents a responsibility shift from the server to the client. People instead focus on the technical differences or the ecosystem.
Gemini transfers the responsibility of designing the appearance of the page from the document author to the Gemini client. The anecdote that prompted this realization: one of my friends and typography nerds was bemoaning the difficulty of doing text layout for different screen widths on his website. I replied, "Text-layout shouldn't be your responsibly as a content creator. Send the ascii, let my Gemini client figure it out." (He has not yet mirrored his website to Gemini, although he remains unsatisfied with its appearance.)
An HTML page represents both content, but also, the style of a page. Fundamentally, it is of course possible to send plain or simple HTML to the client. But doing so is against the spirit of the web, because in the HTML Web system, responsibility for page design lies with the author. In the last couple of years, the concept of semantic HTML has become popular, but only as CSS has progressed to the point where HTML is no longer needed for styling. In the "old-web" HTML was used for styling, so anyone that proposes "classic" HTML as an alternative to Gemini is not addressing this crucial issue. The common thread throughout the history of the web is that the page author, not the user's web browser, is responsible for designing how the page looks.
I would further argue that it is impossible to design an open network that shares responsibility between the server and the client. You might be able to make the web better by using an ad-blocker or reader-view or some other extension. But you've created a conflict of responsibility. If a website is broken, you have to disable your extensions and try without them. Only if the website remains broken can you say it is the fault of the website owner. On the other hand if a Gemini site is broken (in appearance), that is 100% of the time the responsibility of that particular Gemini client.
Hopefully the advantages of having the client in control of rendering are clear, as they have been well enumerated by Gemini proponents in the past. In short, all documents look the same, the user can control their font size and colors without disadvantages, etc. And of course, the content creator doesn't have to worry about it.
Notice how nothing said here references the relative complexity of Gemini and HTML. This isn't a complexity argument. Your Gemini client could decide to, for some unknowable reason, create a dynamic three-column layout or render a document as a GPU-accelerated simulation of a piece of paper flapping in the wind. I would argues these are still within the spirit of Gemini because that complexity exists only for a specific Gemini client.
I would like to make one last note. The web has shown us that most writers and companies can't handle the responsibility that the web requires them to take up; that is, of designing their own site. The vast majority of smaller blogs, personal websites, and the websites of non-tech companies, are not enjoyable to use. (And not just because of bloat or complexity or moving picture ads or trackers.) These smaller websites, in an effort to stand out, frequently make questionable design decisions about things as fundamental as font and color and line-width.