All of those companies whose hardware failed outside of Japan have plenty of other business interests to keep them going. Sega and SNK were primarily arcade machine manufacturers; NEC have an extensive consumer electronics business; and Nintendo have never had a console that did well in Japan but failed elsewhere (the Game Cube, though lacklustre, isn't a failure, and the Virtual Boy failed everywhere).
To suggest that these companies were kept alive purely by Japanese sales is to ignore a significant portion of their revenue streams.
More importantly, we can look at those companies now. The Saturn and DreamCast were only really successful in Japan; Sega subsequently stopped manufacturing hardware and have merged with Sammy, who want to focus on the arcade market.
SNK produced the Neo Geo and the NG Pocket range, only popular in Japan; they went bust, were bought out, and are now producing arcade games for Sega's Naomi board.
As far as I know, NEC no longer produce videogame hardware.
Only Nintendo, who have done at least reasonably well in all territories with all of their hardware, currently manufacture consoles.