"You're going to have to eventually" is the single worst argument for upgrading EVER.
One, you don't know that. You can only guess. People said the same thing about Vista and DX10. The only game I can recall being a DX10 exclusive is Halo 2, and that was clearly a marketing stunt by MS. Hell, it was even cracked to run on XP.
Two, it's a self-fulfilling prophecy. Why are you upgrading? Because the new software is going to be exclusive to it (maybe). Why is the new software exclusive to the new OS? Because enough people upgraded. It is, and always has been, in the consumers' power to tell Microsoft and any developers sucking at their teats to screw off. We don't have to let them shove it down our throats. Again, I point to XP/Vista.
Third, even if reasons 1 and 2 fall flat, and it really DOES become absolutely necessary to upgrade (I'm going to go through my library and see how many absolutely won't run on XP, I bet it's an extremely small number), THEN it is time to upgrade. Shelling out money* now for "might" and "maybe" later is just stupid. There is nothing stopping you from upgrading later when you KNOW it's going to be a good choice.
*And this is the very reason 10 is being offered for free for a limited time. MS knows this bit of logic is a hindrance to their desire to kill off the traditional desktop for the wonderful new world of multi-device, cloud, apps, and every other trendy buzz word that'll make them a continuous stream of cash, so they are trying to stamp it out. You don't honestly think MS is giving 10 away for free just because they are swell guys and are being charitable, do you? Gee, where have we seen that business strategy before?
Well, that was a little more rant-y than I intended when I started writing it. Also, let none of this be construed as a flat out argument against windows 10 (except for that last bit about an OS designed to continually make them money instead of just being a good product). If you're truly excited by windows 10 itself and want it for its features, then by all means. I already stated I'd get it on my windows 8 laptop (probably) because it's in all likelihood an improvement. Just don't fall for the trap of feeling forced to upgrade. If that's your only reason, it doesn't make any sense to do so until it turns out you ARE forced.