The American public deserves the opportunity to vote for Sanders, Trump, and whatever hacks the two elitist war parties select.
If the Republican Party chooses a candidate other than Donald Trump at its convention, the possibility that Trump would declare a third-party/ independent candidacy is already mainstream. This would create a three-candidate race for the presidency in which the independent actually has a chance to win, given the monumental disgust of the voting public with the corrupt, elitist and profoundly venal parties.
So what's to keep Bernie Sanders from declaring as an independent candidate for the presidency? While Sanders has mouthed the expected platitudes about supporting Democratic Party hack Hillary Clinton, his ardent supporters--who almost by definition have zero loyalty to the elitist, permanent-war-is-wonderful Democratic party--might insist Bernie hold true to his independent roots and run as an independent.
That this has never happened before doesn't mean it couldn't happen. Granted, the American system is rigged in favor of the two parties; independents would face major hurdles in getting on the ballot.
The American system has essentially no room for the will of the people: after the party machinery filters out everything but support for elites, vested interests, war and some hot-button social issues to distract the masses from their servitude, there is nothing left of the people's will but a fetid trickle of raw sewage.
It is no accident that party membership and loyalty is in structural decline. The parties only represent various groups of insiders and vested interests, most sucking their wealth off the bloated bureaucracy of the federal government. Party loyalists are by default those who benefit most from the status quo; the disaffected and marginalized have no voice.
The system is rigged to eliminate the will of the people via rigged party conventions. The status quo (i.e. the few benefit at the expense of the many) will defend its perquisites by rigging the conventions to select a suitably venal party hack (Clinton, Cruz, et al.) and then cranking up the propaganda money-machine to whip the remaining party loyalists into voting for the party hack, not because the hack is a worthy candidate but because the party elites say so.
Here's what no mainstream pundit dares discuss: the two-party system is sick unto death. Not only could the two-party system collapse, it should collapse. Nobody can say this in polite company, but reforming the corrupt two-party system is impossible, and so the only way forward is collapse.
Thomas Homer-Dixon described this dynamic in his book The Upside of Down: Catastrophe, Creativity, and the Renewal of Civilization
The American public deserves a real choice, not the rigged non-choice between two party hacks. The American public deserves the opportunity to vote for Sanders, Trump, and whatever hacks the two elitist war parties select.