I am slightly upset at Apple for not supporting iPad 1. I have an iPad 3 but I know sooner or later I will be in the same boat as everyone with an iPad 1. What if I never had DailyMotion on my iPad 1, could I download it? No, because it doesn't run on iOS 5. We should be able to download previous versions.
Also, eventually, your iPad will only be able to surf the Web. All apps will be updated to iOS 6 and above sometime.
Lets say you have Pages on your iPad 1. But Apple updates it to iOS 6, you can still run it on your iPad 1. However, should you connect your iPad to your Mac, sync, and accidentally update all apps to the latest version you would be screwed (Unable to go to the previous version)
I would say you have 10 years or more for Web surfing on iPad 1 if you dont mind it crashing frequently. If you use it for the apps, maybe 1-2 more years max. If you get 4 years out of the device with most apps available on the app store, consider yourself lucky.