Yep - even if one assumes that some of those "never that guy" voters would come around, hold their nose, and vote for the nominee, if even half of them just stay home, not only is the White House a loss (which really, had been the case for some time now), but all those downballot races take a hit; the Senate goes goodbye, state houses slide D, and the Dems make gains in the House.
I don't think the Republicans have a truly good option at this point.
If they nominate Donald, he dumps filth all over their brand and downballot races. It doesn't even matter if he wins or not, because the establishment will be wrecked as he turns the party into the Party of Trump. And the party likely splits, because parts of the big business and religious right hate him with a burning passion.
If they nominate Cruz, the so-cons and the other Trump-haters stay on board, but now Trump is going to throw a tantrum, take his 30% of the base or so, and go home - whether he runs as 3rd party, starts an "anti-republican" movement or whatever almost doesn't matter; whatever he does, its a fractured party and damage to the downballot - negative coattails.
If they somehow manage to nominate someone else - Ryan, Romney, Kasich, whoever, the entire populist wing of the party will scream to high heavens and likely raise a massive hue and cry that will break the party even more surely than the above two options (they'll likely have alienated both the populists and the "true" conservatives). I suppose there a slim chance of finding a real compromise ticket, somehow. (The reanimated corpse of Reagan, with Ivanka for VP?) If they can do that then "all" they have to deal with is a party that isn't openly at war with itself and is covered in filth from the primary season.