Closed primaries might be acceptable in a nation where more than two parties were able to play on a level field.
Ever since third party candidate Ross Perot performed better in the debates than Bush 41 and Bill Clinton combined in 1992, it became illegal for third party candidates to attend let alone participate in debates. I recall Ralph Nader being threatened with arrest when he showed up at a debate and Jill Stein being arrested when she showed up, and
During the past decade some states (California and Washington, for example) started having "top two primaries" that (for all offices except the Presidential ticket) limit the general election ballot to only the top two primary vote getters, which has invariably resulted in two D's, two R's or a D and an R on the general election ballot.
We have ever increasing voter suppression, AND we have ever increasing candidate suppression.