1st. There has been a good look at the historical rate of party switching and the evidence of this 'Bernie Bro' (T.M. Hillary 2016) defection happenned in any significant number above that historical average can't be found. But yeah, you were on a posting page so i guess what? Evidence? Ok. Nailed it Jeff. 538 has done some good number crunching on this vote shift for 2016 but don't let em fool ya. Fake News! 2nd: you should look into the origen of 'Bernie Bro'.
3rd: the ridiculousness of your argument that it's ok for the party to rig the primary Jeff. Seriously? I'm as cynical as the next guy but for all the faults of the republicans (a long list to be sure) one thing that's interesting is that they don't mess with their primaries to the level of the Democratic Party. They leave the fraud and grift for the general election. You wanna know why they don't? Because it angers a certain percentage of their base. Democratic Party leadership, on the other hand, has been carving off chunks of it's left flank for decades now.
4th: Bernie ran on the democratic ticket. If you want a protect at all cost rule to be a rule then enforce it Jeff? If that's the case ( in Jeff world but I'll play along) then why let him into the race from the start?
One possibility was that he could be used as a foil for Hillaries anointing. My guess is they let him in believing they could present a farce as a real competition but, suprise suprise, what they got was, in fact, a real competitor who's ideas and positions were very popular. At that point they could have fought a real primary battle or choose to rig the primary. They chose to rig the primary. They leaks only confirmed what Sanders supporters knew. Indeed, as many down ballot progressive primary candidate knows. It's all about the money and the DNC is there to protect that. That is the North Star of the party leadership Jeff. It guides their every decision and will be our neoliberal tombstone if we don't wrest control from these classist greed heads. Your 'they godda be party members' rule is nonsense. They have primaried good progressives with republican candidates that reflagged themselves democrats for DCCC support. Your rules are just that. Yours.
It's like climate denial. Hillary supporters just can't admit what a lousy candidate she was. Your simply enabling a party that needs serious reforming.