Every website you create, just like any form of application, is based on solving a particular problem. How you solve that problem can vary from dev to dev. Some devs may disagree you your solution just as you can disagree with theirs.
I do stand that htmx can be used to solve a good chunk of problems that otherwise would be done with React, or angular, and so on. Personally, I think htmx makes the problem easier to solve. Well, once you get over the learning curve of the htmx way.
To be able to build a website or SPA without writing much client code (JS, etc) is a win in my book. You don't need a dedicated front end team for larger sites. Also, the designer/UX team (if you have one) can focus on the server-side template system.
Regardless - frontend is not dead. htmx, afterwall, is written in Javascript. htmx doesn't stop you writing javascript code. It's just I hardly need it for business logic. If anything it might be used to compliment CSS.
Would I use htmx if writing a web-based game? probably not. I guess it would depend. I would assume I'd be writing a fair be of javascript for WebGL-based rendering and typical update logic (input, physics, health, etc)
Maybe WASM is a better choice in this domain. I don't know.