For a start, it doesn't have to be a replacement. You can progressively add it in. I work at a very very large organisation with a multi-million line codebase and we splash htmx here and there where it is useful (and where a full blown SPA would be too much to set up). We don't have to ditch any other FE tooling in favour of htmx - htmx "just works" nicely alongside everything else.