Somebody: Sure, but we're dealing here either with prospective coalition junior partners or with minority opposition parties. And these have been among the most successful of new parties in the industrialized world. Beyond that, you have preexisting parties with new names, like the Canadian Conservatives. Given the American political system's inherent two-party bias, the only way a party like the Greens could really become significant here would be by either regionally or nationally displacing one of the two parties, something that hasn't happened in 150 years.
All in all, not only is bourgeois democracy not characterized by periodic revolutions, it's typified by increasingly infrequent turnover of the existing parties. Few revolutions, few strikes, few coups, few assassinations, few civil wars, few new political parties - the capitalist world really is quite peaceful these days, despite the present depression.