SSB is actually already a decent example FOR them not becoming obsolete with each following title. Each title is different enough to the last for there to be plenty of people who have good reason to play the earlier ones. Whether it be physics, items, moves (both differing moves and the moves behaving differently), levels.
I completely agree with this.
Warp, there aren't only a few small tournaments going on; there are many taking place each day. There used to be an entire board dedicated to SSB64 on Smashboards until recently, when they decided to integrate Melee with it. I've been in the SSB64 competitive scene for 5+ years and I can say for sure that Melee does not obsolete it. Sure, there are more options, characters, and stages in Melee, but in the original the physics are much different and the combo possibilities are endless. If someone was playing Yoshi as his/her main character in the original and switched to the Yoshi in Melee, he/she would be very surprised at how much different it is controlling him.
If none of this convinces you, go ahead and play both games with some friends and see how different the experiences you'll get are.