No browser is perfect in every way, but it's a stretch to call Safari a "junk" app.
Safari reignited the "lean and mean" browser wars when browsers were getting bloated and still forging their own paths, not following an agreed-upon set of web standards. Mozilla was trying to establish some kind of web standards, but with just two major players in the market, Microsoft still held the cards. Enter Apple, and suddenly it's two votes against one. Standards eventually win, and Internet Explorer eventually met its maker.
Chromium was born from the WebKit engine, but then eventually forked its own browser rendering engine, Blink. There's still collaboration between the WebKit and Blink teams, code sharing, bug fixes, etc.
Then there are Firefox and Opera, with each trying to have their own rendering engines. Opera eventually tossed in the towel and now uses Chromium's Blink.
Mozilla released Firefox as a lean-and-mean competitor to the original Safari browser. Over time, it re-grew its bloat and became slow and sluggish again, much like its ancestor, Netscape Navigator. In recent years, it has fixed its bloat problem and is a fast and more native browser on Macs.
You mentioned that Safari supports "no extensions", but isn't it a shame that extensions are needed for a web-browsing experience at all? Shouldn't the blame be placed on the fact that such extensions must exist at all? I personally have zero extensions installed and have no limitations. Safari has many extensions, but just not as large of a library. That's not a flaw of the browser, but of extension developers.
You mentioned that Safari doesn't render some websites correctly. So is Safari to blame, or are the websites to blame? The bloat in browsers has often come from trying to be compatible with every website under the sun, whereas Safari tends to adhere closely to web standards. So maybe the websites need to be fixed?
I know what it's like to have a jaded view. I cringe at the thought of using MS Windows or anything Microsoft, but I recognize that some people look at it from the opposite angle where using Windows is the *only* option. I view Windows as "dirty". Could it be possible you have a jaded view of Safari that is not rooted in reality? Do you view Safari as being dirty, and poorly-coded?
As a professional web applications developer, I test my code across Safari, Chrome, Firefox and Opera, and there are subtle differences in web-standards support, but for the most part, they are all fairly equal. Occasional bugs pop up, but nothing show-stopping. If I do encounter an issue in my own code, I can address it. If I encounter an issue with a 3rd-party site, I just choose a browser that works. For example, I have a heckuva time signing into Engadget with Safari... no idea why! So I just use Chrome for Engadget. That's a flaw in Engadget's code, not a fault of Safari.
Nobody is forced to choose a single browser for every site. That's flawed thinking. If you applied that type of thinking to every application, we'd have just one giant application trying to do everything, and it would be a nightmare. So, use a mix of browsers just like you use a mix of applications.
I have tried having other browsers as my default, but keep coming back to Safari. The Keychain integration was a large factor in that, but I still just prefer the "feel" of the browser overall. Just my personal preference, and I don't need extensions fixing the browser for me. It does everything that I need.