Put yourself in Bernie's shoes
Many of the comments I have seen here have really reflected people's hopes and aspirations (in some cases bordering on delusional), but I think it would be useful to put ourselves in Bernie's shoes and ask what we have done. I will present the options as I see them.
1) Do not endorse Clinton before the convention vote. Carry on. Go through the motions, lose. Endorse Clinton after losing at the convention. This might not have been a bad option, but he would have probably lost the platform concessions he was able to get. These are not worth much, but they are something.
2) Go through the convention, lose, and run third party. The likely scenario is that he would get about 20% of the vote in the general election, maybe 25%, and Trump would win the election. Those that think he would win running as a Green are guilty of magical thinking. Sanders really does not want Trump to win.
3) Go full bore on election fraud. I believe that there was serious election fraud in the 2000 and 2004 elections and in a number of governorship elections, senatorial elections and congressional elections as well as this election. However, if Bernie suggested that some of the Democratic primaries were rigged, the corporate media would laugh him out of town. They will not take election fraud seriously, because it is outside of the Overton window and proof of election fraud would reveal that the corporate media are really bad at their job. If you want to prove election fraud you have to the goods in triplicate or quadruplicate and they have to be unassailable. Statistical arguments will not cut it. Sanders does not want to be written off as a delusional old socialist so this was not an option.
None of these options are very good so he chose another bad but not quite so bad option:
4) Endorse with a speech that forced HRC to at least mouth the talking points that he has been making, while keeping the campaign alive in case the HRC campaign implodes. This option sucks but it is not as bad as the others from Sanders' point of view. It still keeps a very tiny bit of hope alive, it does not help Trump, and it makes HRC put herself on record as favoring Sanders' talking points. She will still do what she wants, but her speech will at least be on the record.
In the absence of magical unicorns with golden wings, Bernie had several bad choices and salvaged what he could. His body language said he was not happy, but he did what he thought he had to do.
Now a word about the Democratic party. The leadership of the Democratic Party is awful and the Democratic Party has been as responsible for the miserable turn to the right in this country as the Republicans. So does Bernie think that buying DWS donuts will make everything OK. No. Bernie is no fool. However, the Democratic party can change. Right wingers systematically took over the Republican Party starting with the loss in the 1964 election and made it into a different, much worse, party. There is no hope for DWS, Rendell, Reid, etc., but Bernie is calculating (maybe just hoping) that we can change the Democratic Party, making these assholes irrelevant. We need to use our brains more than our emotions and we need to get off our asses at the state and local level to replace incompetent conservadems with more competent people that don't worship corporate interests. Whether this is a better strategy than building up the Green party is not clear to me, but they are not completely mutually exclusive