Simple is a changing target.

In a one person team merging your server with your client and using HTMX can be simpler (depending on if the value is in your UX or in your logic/data).

However as the team gets larger then I find having a clear separation between your API(s) and your Client(s) is much more simple and maintainable. Also it can unblock product decisions like offline mode or PWA's.

Finally its worth stating that being in an SPA community like React can give you access to "economies of scale" in terms of the libraries available to save you time (Tables, animations, UI kits, Video players, etc.) allowing you to do more with less people.