Very convenient way for party insiders to justify coronation of their preferred candidates regardless of "benefit to the nation." This same line of "gatekeeper" reasoning is exactly what justifies polling locations being closed and reopened elsewhere without notifying voters, years-long revolving door media propaganda campaigns, and accusing everyone who disagrees of being a Russian asset. The Republicans and Democrats alike are guilty of such behavior.

I too believe Trump is a symptom, not from lack/failure of gatekeeping, but rather the act of gatekeeping itself. I think the general public is (subconsciously or not) aware that these party insiders view themselves as "gatekeepers" and are anxious to "stick it to them" even if it comes at their own demise. Whether or not you agree or disagree with them, the issue here is that their trust in what they viewed as democracy was already eroded. This is probably reflected through the terribly small number of working class voters who participate in the political process.