This will actually be a short post, as it's relatively simple to explain.
US elections, more than Canadian elections, are driven by turnout.
As such, lets make 100 sample voters. These represent the 100 people who voted in the last election. Along side them are 100 people who, last time, decided to stay home and not vote.
Now I know the turnout in the 2012 US election was actually 54%, but this is an example.
Hillary Clinton is the second least popular person to ever run for President, if the polls are to be believed. The problem is the most unpopular is Donald Trump, by quite a wide margin.
Now lets say that of our 100 voters, 50 are Democrats and 50 are Republicans. Because Clinton is not popular, many of these Democrats may want to stay home, but Trump is so very scary, most of them will probably vote anyway.
Trump however will almost certainly keep some Republican voters home. Lets say 5 of them stay home.
The problem is not that Trump will lose; Trump will lose even if all 5 came out to vote, the problem is downticket.
See elections in the US are grouped on one day. So you may end up voting for President, Federal Senator, Federal House Rep, Governor, State Senator, State House Rep, Mayor, City Councillor, County Councillor, and County Comptroller.
The problem is it's the Presidential race that drives turnout. That means where a Republican Senator could normally count on his 50 voters turning out, only 45 will bother showing up because of Donald Trump.
This is the reason why some Republicans want to get someone else to run. They could endorse a Libertarian, find a Social Conservative already running, find a Moderate to run as an Independent, or even do a patchwork of all of these in various states. The objective is to keep those 5 voters, Republicans who are so turned off by Donald Trump they'd rather stay home than to have to pick between Trump and Clinton - to get those people to get out and vote, not just for President, but for all those other offices.
As such I do actually think that they will find a Trump alternative, but it may simply be someone already running like Gary Johnson, the Libertarian. They may also try to help the Constitution Party gain ballot access in more states as the party is known for being more socially conservative.
While I'm certain some of the people behind this push personally dislike Donald Trump, that personal dislike has little to nothing to do with their pushing an alternative to him in November.