I hear you, but the historic pattern of the 'eight year swing' is on the Republicans' side in 2016.
There's going to be a monumental level of fatigue with the current style of governance by the time of the election. Even lots of Democrats will be weary of Obama and his leftist tyranny by then. That'll tend to dampen Democrat enthusiasm for getting out the vote. Especially if the Dems run another committed Marxist with a thin resume'.
Note the historic electoral tsunami of 2014. If the American people could have voted for a Republican President last year, they would have handily elected one.
And if the Dems insist upon making Grandma Clinton their standard bearer, almost any Republican she faces will soundly trounce her.
I hear what you are saying. That was some of the sentiment going into 2012. Maybe it wasn't as strong as it is today. But somehow, dems turned out the vote AND Republicans didn't turn out bacause of the candidate (Romney). We have a great opportunity to have the same showing in 2016. If Jeb Bush ends up on the ticket, THIS voter won't be voting. I held my nose and voted for McCain and again for Romney. Couple that with the complete lack of platform, passion, evidence, etc. in the GOP camp? If I don't get to vote for Walker, Cruz or somebody that at least has some ligit conservative creds, I will abstain from voting for President. I doubt I am alone.
Now, about underestimating the democrat voter turnout. We do this routinely for Presidential General elections. Look at the elections even where Republicans won. Wins were by screaming tight margins. While Obama won comfortably in 2012, even though the House and Senate went toward Republicans. Explain that away and I'd feel more comfortable about your optimism.