Please take that and run with it. What you saw: WP7 won't be supported anymore. What was said: WP8 won't have backwards compatibility unless the developer makes one for 7.8. Android does it all the time, it's not impossible.
Tell that to my aunt and her new Samsung Galaxy phone. No not the newest, the first edition. She can't sideload apk's on her phone so she can't get the new market, she can't play quite a few current games, so she's not in the loop. So no, your argument is NOT supported by all devices.
I don't deny that obsolete is not in the future for WP7 but it's not happening anytime soon. It's about to happen for the 3GS soon enough, it's happening to a lot of newly released Android devices, and it'll happen to WP7. In reality, the update span has been on pace with iOS. The only people that it should sting a little is Lumia/Titan owners and even then, they see an update.
And yes, let's continue with Android. Remember when Netflix and hulu came out and only a handful of devices were supported? See how long that took for all devices to get a chance? Remember how long it even took for it to come out? There are PLENTY of games that I try to download on my Mytouch that can't work because of my processor. Eventually, fixes became available for the majority of Android's market to work across the board. But don't sit here and pretend right now that it's always been that way. It hasn't. WP8 hasn't even came out yet and you don't know what fixes or substitutions they have in store.