The only downside of Cruz quitting now is that it gives the Republicans (well, those Republicans willing to back Trump) more time to get their strategy together before the general election. If they'd still been fighting it out until July, that would have been ideal for the Democrats. On the other hand, if that had led to a contested Republican convention, they might have been able to pick a candidate who appealed to constitiuencies beyond the pointy-white-hood-and-burning-cross demographic of Trump supporters. So, it would have been a risk for Clinton.
Trump is polling 50% or less of the Republican vote in most places. Pretty much everyone who isn't a Republican man has a very strong dislike of him. He has unprecedented disapproval ratings (though Cruz was close to the same levels), particularly among the key demographics that are generally understood to determine US elections these days: minorities and women. One analyst on the excellent FiveThirtyEight website (the number crunching site for election nerds) estimated a few weeks ago that Trump has about the same share of the US electorate as UKIP does in the UK, about 12-13%. Unless he can shift that, not only is he utterly screwed but he will almost certainly cost the Republicans their majority in the Senate and is even, some people think, putting the Republican House of Representatives majority at risk too.
As with the swing constituencies UK, in the end what matters is who can capture the swing states. Even in good years this is hard for the Republicans because they have gone ever further to the fundamentalist right and as a result their core demographic (less educated, older, conservative white people, particularly men) has shrunk. Since the end of the Cold War, the Republican candidate has only won the popular vote (the largest number of voters across the country) once, in 2004 when War on Terror nationalism was at its height and the Democrats put up a useless candidate. Even then, with all the massive advantages of incumbency, a useless opponent, and the then-popular War on Terror, Bush only just scraped a majority of the vote (50.6%).
After the 2008 and 2012 electoral disasters the Republicans had discussions about how they could broaden their appeal to women and Hispanic voters. Instead, they have Trump: a gibbering misogynist who wants to build a wall to keep the Mexicans out.
This is an interesting analysis from earlier in the week, from Republican and Democrat insiders, speaking anonymously to a Republican-leaning site. 75% of the Republicans - who are the people most likely to have an unrealistically positive view of Republican chances - thought that Clinton would beat Trump in their own state.
So, unless things change dramatically, all the evidence suggests that Clinton will crush Trump like a bug in November, as she would have done Cruz. I don't think there is any serious analyst or party strategist (in either party) who thinks any different at this point.