This isn't some conspiracy by "a foreign power" it's well documented history
But reality: Bernie was really popular with the 18-36 demographic. But they don't turnout to vote in the general election, let alone a primary. So here we are. Old people get their way because they show up.
And I'm not saying Bernie didn't energize voters: just that it's a really high bar to energize that to a point of participation.
What I can say is that in that election the country was looking for something different from the government. They knew that the status quo wasn't working for them and that was the only thing Clinton was offering. I know that in the end that feeling was enough to drive at least some people who normally voted democrat to vote for trump (or third party). The fact that the DNC ignored the will of registered democrats and decided for themselves that Clinton was their Chosen One before the primary election was held also caused some democrats to vote for trump or to stay home entirely. I can't claim that the DNC was the cause of trump getting elected the first time, but they sure didn't help.
That was the one I paid close attention to. If 2016 was anything like it (and I'm sure it was, considering this year's convention tactics were used all the way back in the 1940s to force Truman on us), I have no doubt that this is the DNC's modus operandi. The true steal of the last 3 elections were establishment Democrats' theft of the liberal and leftist vote. And in 2 out of 3 of those cases, they paid in the general.