I don't necessarily disagree that there comes a point at which you are working more against HTMX than with it. I think rather than measuring that contention point between HTMX and SPA/MPA in terms of interactivity I'd measure it in the amount of client side state and logic needed. If you need a bunch of client side state and logic, for example a digital audio workstation, then a SPA would definitely be the appropriate pattern.

But interactivity in largely content-driven sites doesn't necessarily preclude HTMX. You can do a bunch of fancy interactive stuff with HTMX and some minimal JS. Drag and drop sort is a tiny drop-in JS snippet. Skeleton loaders and spinners are supported out of the box in HTMX as is infinite scroll as others pointed out.

I'm not saying Next.js, Solid Start, Remix, Nuxt or some other hybrid server and client rendered model doesn't work for forum software. Or that even client only rendering can be a valid approach if you accept the trade-offs (no curl-ability). People should use whatever they feel productive building with.

But if your backend is already a non-JavaScript server rendered framework like Django then HTMX makes a ton of sense to add some interactivity.