Consider this: you're doing server side rendering whether it's JSON or HTML fragments you're sending across. They're just different serialisations of the same data. The problem that SPAs were meant to fix was that each update of the page meant a full rerender under the classic model rather than just rerendering those bits that had actually changed.

You're still going to be able to take advantage of those client machines to do rendering, but the difference is that with HTMX, you skip serialising JSON, so your clients aren't stuck doing unnecessary rendering. The load on the server is barely different, but the client has less to do.

Now, if you're doing an awful lot of data manipulation on the client, that's a different story.