From what I understand, this is what you are saying (correct me if I'm wrong): Everyone who voted in the republican primary is going to vote for whoever the GOP nominee is in the general election and likewise on the democratic side. Since the republicans have the higher primary voter turnouts, they are going to win the general election.

Now, that's one screwed-up logic!!!

Let's see how things actually work. Look at the data of hypothetical match-ups in the 2016 general election between Hillary, Bernie, Trump, and Cruz. The democrat is always beating the republican on average. As of today, it's:
- Hillary vs. Trump: 50% to 42%
- Hillary vs. Cruz: 49% to 44%
- Bernie vs. Trump: 52% to 41%
- Bernie vs. Cruz: 52% to 39%

http://elections.huffingtonpost.com/pollster#2016-general-election