A political party is free to change the rules for nominating candidates however and whenever it chooses. It is free to nullify the decision of those voting in a particular primary. A political party is free to nominate whomever it chooses [and almost certainly multiple candidates for the same office it wishes should it choose].
Ultimately the party, not a judge, chooses whose vote matters and whose doesn't. Placing the imprimatur of the state upon a political party's process doesn't change that or make the process of candidate nomination little 'd' democratic. The people within a political party charged with making the rules for candidate selection are not elected or selected little 'd' democratically. The process of nominating candidates is not little 'd' democratic in any meaningful sense.
Whenever people talk about alternative voting systems, the consensus seems to be that it would be impossible to implement in the US. But why? What drives this obsession with choosing between two evils rather than choosing among several, where one's own views might stand a better chance?
Theoretically third-party candidates could gain exposure through the General Election debate however, the Commission on Presidential Debates which organize the debate makes it very difficult for a third-party candidate to be eligible to the debate. Indeed candidates need "a level of support of at least 15 percent of the national electorate as determined by five selected national public opinion polling organizations" as defined by the CPD.
You may then wonder who is the CPD? The CPD is a PRIVATE organization financed with PRIVATE money. They claim to be nonpartisan even though it is governed by former chairmen of the Democratic National Committee and Republican National Committee. No wonder why they don't want to open the door to third-party candidates...