Use the right tool for the job. Obviously Gmail is a much better fit for React/Vue/etc than HTMX. Gmail is the perfect use case for an SPA but most web apps don't need to be an SPA.
Is it really, though?
It's a paginated list of emails, some drag-and-drop functionality, buttons for some ajax functionality.
It works reasonably well, but it doesn't need to be so high Javascript compared to Google Docs or maps.