I think the value of markup-based templating approaches such as HTMX and SGML comes from enabling content authors to create dynamic document-oriented sites without having to deal with dangerous tools such as JS and the endless ways too shoot yourself and your visitors into the foot (through eg injections and other security issues). The difference between HTMX and SGML being that HTMX integrates templating as markup vocabulary extension while SGML brings syntactical templating at the markup declaration level. A developer, OTOH, might choose JS-heavy approaches to avoid limitations and learn new syntax/concepts, and to re-use existing knowledge from/to other projects. I don't know that speed of development is of primary concern.