The iPad 1 is a great tablet for letting your kids watch movies on an airplane, but it's obsolete for most every other use. Even my wife, who would still be watching TV on a 13" tube set if I hadn't given it away, finally gave in and bought a last year's iPad air versus putting up with it any longer.
IMHO, the iPads are pretty decent at longevity -- I have 8.3 on my iPad 3 and it's arguably no worse than it was on 7.x.
I think Apple could benefit everybody by doubling (or more) the RAM they put into iPads. New OS releases and expanding app capabilities eat into RAM to the point where you can't keep apps cached in RAM long enough and app switching becomes app re-launching and the inevitable grinding away as they refresh paged-out objects from the network.
IMHO this is what makes them slow/crash/obsolete. I'm sure the iPad 1 with 4 gigs of RAM would still be pretty useful.