Primaries aren’t restrictive

Parties in the US are considered private organizations and party chairs have enormous power, as a result.

Utah Republican Accused of Trying to 'Steal an Election' by GOP

Currently in the courts, because this degree of infighting is cutting across a number of people with real power and influence. But for less high-profile candidates, this is absolutely something a party official can (and periodically will) do, when the party leadership doesn't want a contested primary.

There is a huge problem with campaign finance laws in the USA, despite our many laws regulating it, and admittedly not having a democracy would fix that specific problem

I would argue that having the problem makes the system undemocratic. When you can buy your way onto a ticket and buy your competition off of it, the end voter has far less real electoral choice. And when districts bloat to the size of 600k-700k voters in the case of national House Reps and as much as 40M for Senate seats, the idea of representative democracy is stretched to its functional limit.

How does a pair California Senator seriously represent the diverse views of a state this large and varied? And not even a split pair? It isn't as though you're electing the 1st and 2nd place winners. You've got two individuals who rose to the rank largely based on how much money they could raise from friends in domestic industry. Not based on their popularity or the popularity of their policies in any meaningful sense.