You do realise that you can install any other internet browser on Win 7 if you want.
As far as Vista being a slow pile of poop,when XP first came out it was running on what kind of hardware? It wasn't snappy, it wasn't fast and everyone complained that it was slower than Windows 98 and used twice as much RAM - but hardware caught up. You had Windows XP machines pushed out of the factory that in relative terms were 50 times more powerful. So when Vista was released people had got used to a certain level of snappiness, roll forward to Win 7 and the hardware hasn't improved as drastically but the speed and stability have gone through the roof.
Is it the best? Well some would say no because the one that brought Windows to the masses was Win 95, the one that cemented into the business world was Win 2000 and the one that unifed them all was Win XP. Win 7 does have more and better than XP but it also has such entrenched support that it is difficult to cannibalise it's own market.
As far as Linux goes, well I wholeheartedly welcome other free OS's to the fold. They have plenty to offer but if they are to become popular and dominant they need to surgically remove the jerks that think they can rule the world as the unfinished product they are.
http://www.theregister.co.uk/2010/07/23/rising_cost_of_scaling/
This is a perfect example, but it also shows the problem with Linux for the end user.
Facebook uses a ton of open-source software. While ostensibly free, to make projects like Linux work for its purposes, Facebook heavily customizes them. While the company may not buy as much software, it ends up writing or customizing quite a bit of code.
So the same applies to the user. There just aren't any programs out there written for Linux. Using WINE you can get them to run, but at what performance? Games can install, but without DX they are running slow as hell. Linux may well have a case to complain at Windows but people in glass houses shouldn't throw stones. Clean up your own failing first and that ivory tower will look a lot less hypocritical.