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.
I personally think that if you are adding a sortable component to a content-driven website, maybe it is moving away from being such a website. Normally a textarea should be enough for most people to order a "list" but if the list items have ids (they are entities), then that'd not suffice, and you may be going in the direction of a web application.
Otherwise, I'd recommend HMTX even if you have a super flexible JS backend.
That said, this is all baseless thinking without seeing any requirements and as I said before, trying to choose the right abstraction is the hard but fun part.