You forget that MetaKnight and Snake are in a tier of their own in how extremely popular they are.
Fox, Marth, and Sheik were top tier, but that doesn't at all discount the fact that Falco, Falcon, Peach, Samus, etc were all
almost just as good.
Take for example Shades of Falco --
http://www.youtube.com/watch?v=7vHxXE6VmJE
DaShizWiz completely ***** in that combo video.
In Melee, the characters who were good had a relatively small difference in potential. When compared to Brawl, there's a huge gap in ability between Top Tier and High-Tier. That won't change either because Brawl is completely based on character specific ATs. Melee had a smother transition between tiers because comboing was possible between characters in Top and High Tier and ATs were globally present to all characters and not character specific.
Also take for example there were many numerous people who were incredibly good at Melee bottom tier characters. Gimpyfish comes to mind and was an excellent Bowser player and t!mmy and tommy were excellent Pichu players. I don't think any one Captain Falcon main has arisen in Brawl and claimed numerous wins in tournaments as of yet.
In Melee, bottom tier was viable. It's hard to consider Brawl Captain Falcon viable since he's at bottom of the bottom.