Akira, the only flaw in your argument in regards to the competition that Nintendo has beaten (in the handheld market) is that all the firms - SNK, "Tramiel" Atari, and NEC were incompetent underachievers in the other areas of the console market as well. Neo Geo, 7800, Jaguar, PC Engine etc - none of their products dominated any market they entered.
Sega was the only competing firm with some success - and then only with the Genesis which still ended up being superseded by the SNES. All its other products, the Master System, Mega CD, 32X, Saturn and Dreamcast were failures.
Sony by comparison has crushed all before it with the Playstation 1 and 2 to an extent that Sega never did. It is for this reason alone that I think Nintendo are finally going to come up against real competition and lose their dominance in this market as well.