Bush was never going to be the GOP nominee.
We'd be saying President Cruz or President Walker right now if Trump hadn't ran. Maybe Rubio but I doubt it since he reminded too many GOP voters of Obama.
Where would that candidate get funding from? Jeb had the funding locked up.
If you want to talk about free media, none of the candidates had any free media appeal besides Christie, who was damaged by bridgegate. The media likes blunt, open talkers who have interesting events in their lives.
This is how desperate they were at the beginning of the race for media attention:
http://www.nydailynews.com/news/national/stasi-donald-trump-foes-turn-shameless-desperation-article-1.2314946
Obama was able to use his bernie-style grassroots organization to raise a comparable amount of funding to Hillary in early 2007. Not even Cruz's grassroots network was able to raise funding to levels anywhere near Jeb's.
Funding doesn't mean jack if you can't get votes.
Jeb was never the "front runner". Polls had it as a three way between him, Walker, and Rubio before Trump jumped in. Walker would've had the backing of the Koch brothers and a decent amount of the grassroots. Cruz would've had trouble with funding but the base of the Party would've been largely in his corner.
Jeb was the easiest punching bag in GOP history. Walker, Cruz, Christie, Rubio, etc. would've hounded him to no end. The base of the Party didn't want another Bush, and they definitely didn't want somebody soft on immigration. He had no realistic shot.
It was basically a 2-way race between between Bush and Walker in the beginning. The problem is that those candidates actually agree with Bush on almost all Bush policies except for a couple like common core. What would they attack him on when they actually agree with almost all of his policies?
Trump was uniquely positioned to attack Bush, because he attacked Bush with third-party candidate-style rhetoric.
For Walker, it was a catch-22 situation, the Kochs were extremely reluctant to heavily fund him unless he supported the Koch immigration plan:
https://www.bloomberg.com/politics/articles/2015-04-21/scott-walker-breaks-with-the-kochs-on-immigration
But if he had been weak on immigration, there wouldn't have been a niche for him to differentiate himself from Jeb as you point out.
Rubio was right there with Walker and Bush in the polls in the very beginning right before Trump jumped into the race.
If Walker didn't read the electorate and move right on immigration then he would've went down the Bush path. Cruz would've been the only alternative for the base to coalesce around. Cruz would be the nominee winning with a Goldwater-esque campaign. This GOP was not, under ANY circumstance, gonna go for a candidate that was soft on immigration and they were especially not gonna vote for the penultimate establishment figure with a Mexican-American wife. It wasn't happening for Jeb.
Walker might have been able to tow the line between the Cruz and moderate wing but if he didn't shift on immigration then he to would've fell. Cruz is the only person left. And given that Cruz was the runner up this time around then it's perfectly reasonable to suspect that he would've been the nominee. Let's not kid ourselves here; Cruz was the number one 2nd choice for Trump voters.
We saw the early signs in the 2012 primary with people like Cain and Bachman leading the polls at certain points. The GOP base wasn't gonna have another establishment Republican be their nominee.
