Windows is not going to slow down over time any more than OSX, with the caveat that you are a user that knows what he's doing. The reason that PCs get this reputation is because Windows users tend to install a lot of crap off the internet on their machines (because there is more crap available on the internet that's actually compatible with their machines). And up until recently, Mac's Safari didn't even support plugins, while Windows's IE has been more than happy to let you slow it down with too-easy-to-plug-in crap for many, many years. If you know how to keep your system clean, neither OS is going to naturally slow down over time.
You cannot future proof against gaming. Not if you mean modern, high-quality graphics games. Those will always look great only on the latest (0-1 year old) graphics cards, be playable on somewhat older cards (2-3 years) and probably be unplayable, except for low-quality-mode, on anything older (4-5+ years). If you decide on an iMac, you basically add +1 year to the age of your graphics card because they use laptop cards that are essentially the performance equivalent of a 1 year old desktop card.
But for future proofing everything else, iMac is definitely a good choice. Apple's quality and support ensures the system will probably still be in good working order 5+ years from now, whereas PCs, tending to appeal to the cost-conscious buyer, are often made more cheaply and struggle to last.
But if you don't mind the hassle of upgrading, you're always better off buying a not-top-of-the-line system every couple years than buying a top-of-the-line system and keeping it a long time (because the average annual cost works out the same while average performance will be higher).
