Which voters? Trump voters? If Trump wins a majority of the actual votes cast in the primaries, I think he should be the nominee even if he doesn't have a majority of the delegates. But if he doesn't have a majority of the delegates or a majority of the votes, then it is completely fair for it to be up for grabs.
After all, Lincoln himself won the nomination even though he didn't have the most delegates at the convention. Seward did, but he'd ticked off too many other Republicans, and they weren't willing to give it to him. Lincoln was the compromise nominee.
In terms of the "GOP-E" giving it to someone besides Cruz, I think that'd be tough. If Trump loses, it will be because poor organization at the state level meant that many of his delegates weren't loyal to him. But Cruz excels at that, and his delegate are much less likely to defect. It'll be hard to rally delegates around some other candidate if Cruz' folks stay loyal.
If the process were done honestly and fairly, then I would agree. But we all know that's not what would happen. ;)
Besides, Romney didn't have a majority either. And so many shenanigans were pulled to make him the nominee.