Bernie arguably had more supporters than Hillary - he had millions of voters not "qualified" to vote a Democratic primary. If primaries were open or if American Elections were more than a two party race regardless of allowing third parties to run, he may have well won the nomination and general election.
I think that if all the primaries were "open" primaries-that is, the primary voter can just go down and vote in the primary without pre-registering as a member of the party-then Bernie might well have gotten more votes in the primaries. As to the 15% of the vote that goes to party leaders, we don't know if Bernie's victory in the primaries, (if he would have won that), would have been enough to "flip" the party leaders over to him. Starting out, there was pressure on the party leaders to vote for Hillary because of all the time she had worked for the Democratic Party. If the primaries were all "open" and Bernie won the popular vote in the primaries, there would have been some pressure for them to go with the will of the primary voters and "flip" over to Bernie.
Maybe yes, maybe no. And I am not sure that "open" primaries are necessarily a good thing. However, it is conceivable that under those rules, Bernie might have taken the nomination-he had a much, much stronger run than anyone expected.