The imagined use-case for Pushup is the same niche that Rails, Django, and Flask apps occupy, which is a wide range but broadly I would describe as page-oriented, HTML-first, server-side frontends.
I started Pushup because I asked myself, I like programming in Go and think (like you) it's great for web backends - but what is holding it back from being a great full-stack web dev language or environment? net/http is a great building block, and html/template is solid, but I felt more was needed. Like opinionated project directory structure, for one. But also the world has changed since 2006 when Rails and Django debuted.
Obviously, we live in a world in which React and the SPA is seemingly dominant, but projects like htmx have recently shown that you can achieve modern UI/UX in the browser on a server-side app, which less JavaScript. BTW, more and more JavaScript frameworks, like Remix, are (re-)discovering the value of SSR.
I was also motivated to give an old-school mod_php feel of, just add a file and you automatically get a URL route. That's a nice developer experience but also good for long-term project maintenance.
Finally I was struck by this essay[1] about the demise of the "mildly dynamic" site. There is a wide diversity of types of sites and apps, from quick experiments and prototypes, small-to-medium sites of many kinds. But also since it's Go, I think it could scale up on performance to large-scale sites, and as a static binary, be nicely suited to edge apps like fly.io.
[1] https://www.devever.net/~hl/mildlydynamic