This happens to most consoles. Classics are going to be re-released if they have an audience. I mean Shadow of the Colossus, Ico, Devil May Cry, Kingdom Hearts, Sly, God of War, Ratchet and Clank, Okami... are all examples of ps2 games that have been rereleased on other consoles. Doesn't make a console obsolete.
GC still has great exclusive games. Baten Kaitos, Star Wars Rogue leader, F-Zero, Double Dash, Melee, Sunshine, Batalion Wars, Star Fox Adventures, Super Monkey Ball, Luigi's Mansion, Eternal Darkness, Wave Race are some favorites that come to mind.
Also, it's not just exclusives that matter. I love Killer7 for example and it's a lot better on GC than on PS2. Besides, I like playing games in their original format and the Gamecube controller is awesome.
edit: multiplats most of the time also looked better on GC compared to PS2
Spending warm summer days indoors
Writing frightening verse
To a buck-toothed girl in Luxembourg
NES, SNES, N64, GC, Wii, WiiU, GB, GBC, GBA, DS, 3DS, Mega Drive, Game Gear, PS1, PS2, PSP, XBOX 360, Atari Lynx
