Kasich can't win either even if Cruz dropped out.
The problem with Cruz dropping out is that if he did, the anti-Trump vote would have no distractions or decisions to make. You vote Kasich or you vote Trump. At that point, the establishment would not have to hedge their bets between the most viable candidate to stop Trump and the candidate they like the most.
The character of the campaign would change without Cruz, as would the news coverage of Kasich, who would become the candidate who had to be interviewed, discussed, and inevitably praised for his moderate views and electibility. Even (especially?) as a Trump voter, I'd prefer to keep Cruz in the race.