If Rubio were to back out it would bolster Cruz. If Cruz were to back out it would appear to bolster Trump as the majority of his voters prefer Trump as a second choice. It appears that he may now have the plurality he needs since Trump is pulling in about 25% more voters who never vote, never pay attention, and who know absolutely nothing about politics or policy or ideas or basically anything. They're just mad and he's saying angry things. They like that. This is the "Trump" effect.
He has riled the perpetually angry and least intelligent, least educated white electorate in this country who demonstrate the strongest tendencies towards racism and xenophobia. That is the core of his support. It has appeared over the last three primaries that this base is increasingly likely to be capable of achieving a plurality within the GOP even if it's hopeless in a general election.
Then again, I could be wrong. Trump's disfavorability remains at 60% within the GOP itself, and none of his wins repudiates that ceiling within the general GOP electorate.