Steam is great as an idea.
But its technical aspects had already become outdated by 2010. It’s mainly a (slow) web browser.
I mean.
Purely technically, you're not that wrong - the desktop part of it
is a web browser. A Chromium-based custom client with no address bar, designed to browse the Steam ecosystem. (with the sole exceptions of the Library and Downloads pages, which are client-specific and you can't access them from the web)
I'd like to see you try to come up with a better way to keep parity between the web and client-based storefront versions though.
Like, it's all designed to be accessible from the web. Storefront, community, account settings, etc, everything but the actual installing and playing of games. You can buy games on Steam without opening the client, and then install them in the client without ever touching the 'web browser' pages.
And of course everything else it does goes far beyond what a web browser can do.