Rockefeller winning the nomination isn't implausible, what killed his chances was the fact that his wife Happy gave birth just shortly before the California primary; something which brought back up the divorcee issue and ultimately sunk Rockefeller's nomination aspirations.
It is true that without Happy giving birth, Rockefeller might have won the California primary, but that is very different from saying he would have won the nomination. Even with California--and yes, I know it was a winner-take-all state--Rockefeller would be far short of a majority.
One reason: Goldwater's support in the South. In OTL, of the 278 southern delegates, 271 voted for Goldwater, and Goldwater's southern coordinator, John Grenier, claimed that 260 of them were "rock solid", meaning that they would have stayed with Goldwater even had he lost in California. George Gilder and Bruce Chapman, *The Party That Lost Its Head*, p. 184 (the book was written when both Gilder and Chapman were liberal Republicans). Goldwater also had the advantage that whereas he had virtually unanimous support of delegates in southern and some western states, even the most "liberal" northeastern states were not solidly anti-Goldwater. There were at least *some* absolutely unshakeable Goldwater delegates almost everywhere. Even Massachusetts contributed five Goldwater delegates--including the man who had been campaign manager for Robert Welch in the latter's unsuccessful 1950 campaign for lieutenant governor...
In OTL the vote at the convention was as follows:
Barry Goldwater - 883 (67.51%)
William Scranton - 214 (16.36%)
Nelson Rockefeller - 114 (8.72%)
George W. Romney - 41 (3.14%)
Margaret Chase Smith - 27 (2.06%)
Walter Judd - 22 (1.68%)
Hiram Fong - 5 (0.38%)
Henry Cabot Lodge, Jr. - 2 (0.15%)
https://en.wikipedia.org/wiki/Electoral_history_of_Barry_Goldwater
California only had 86 votes, so that if Goldwater had lost California, he would still--if other delegates had voted the same way--have had about 140 delegates to spare. Now it is true that some delegates who were undecided and in the end voted for Goldwater might not have done so had he lost in California. But that enough would have done so to cost Goldwater the nomination is questionable (after all, the California primary was late in the season, and most delegates--even those not formally pledged--had probably already made up their minds, or at least would not be dissuaded by what after all would be a *narrow* Goldwater defeat in California.) And what is much more questionable is that enough of them would have been persuaded to vote not only against Goldwater but *for Rockefeller* who was absolutely hated by the Right and who everyone knew would be at least as divisive a candidate for the party as Goldwater.
If Goldwater loses narrowly in California, it is quite plausible that he will still win the nomination. It is also fairly plausible that he will fall a little short and the convention will seek a compromise candidate who will at least be acceptable to both liberal and conservative Republicans--the most obvious name being Nixon. Now I know that many people think that Nixon would not be willing to take the nomination in 1964, that he saw that LBJ was going to win, etc. The evidence is not so clear, however. Let me quote from an old soc.history.what-if post of mine:
***
Nixon's behavior in OTL 1964 does not seem to me to be that of a man who
was convinced that the GOP was sure to lose and would not take the
nomination under any circumstances.
Again to quote Gilder and Chapman (p. 154):
"Many critical observers pointed to evidence during 1963 and 1964 that the
official Nixon posture of aloofness and neutrality was being stretched to
the point of deception. Their evidence suggested that though Nixon might
realize that he could not court the party, that it must court him, he
nonetheless did actively seek to stimulate its ardor directly and
indirectly."
Gilder and Chapman note that Nixon made overtures to the Goldwaterites
before the California primary--which he, like most observers, thought
Rockefeller would win. ("Three days before the California primary, he
privately predicted a Rockefeller victory and told friends he was ready to
heed a call to service."--p. 137) He periodically denounced "stop-
Goldwater" efforts.
Then, after the California primary, with Goldwater the clear front-runner,
Nixon observed in a speech at Detroit that Goldwater was, after all, still
not nominated, and that new opposition could develop at any time. If a
deadlock did develop, and "if the party is unable to settle on another
man," he would willingly accept whatever role the party might ask him to
take. "And if the party should decide on me as its candidate, Mr. Johnson
would know he'd been in a fight." Two days later, at the governors'
conference at Cleveland,
"Nixon attacked Goldwater on the very issues for which--in a unity ploy--
he had defended him two days before in Detroit--the United Nations,
diplomatic relations with the USSR, social security, right-to-work
legislation, TVA. 'Looking to the future of the party,' he declared, 'it
would be a tragedy if Senator Goldwater's views, as previously stated,
were not challenged and repudiated.' (This was the man who shortly after
called Scranton a 'weak man'--because he changed his mind so often!)..."
(pp. 151-2) He then went on to meet with Michigan Governor George Romney,
and Romney claimed that Nixon urged him to run.
"Besides wriggling in and out of other people's ideologies, Nixon went
well beyond his pose of strict aloofness in Nebraska and Oregon by
communicating directly with his managers there. According to an Evans-
Novak account at the time, his neutrality posture also did not prevent him
from attending a strategy conclave of all his main supporters, presided
over by former Secretary of the Interior Fred Seaton, on May 30 in New
York's Waldorf Towers. Anticipating a Rockefeller win in California's
primary, Nixon scheduled an eleven-state speaking tour to follow it. The
kickoff was to be a testimonial dinner on Long Island for Congressman
Steven B. Derounian and the guest list featured a half-dozen top
Republican leaders who had not committed themselves to either Goldwater or
Rockefeller. The facts do show that *at the very least* Nixon cooperated
with his supporters and arranged his plans so that he would be within
earshot if the call to duty came."
(My apologies for relying so much on Gilder and Chapman's book; it is both
biased and dated, but it's the only detailed book about 1964 I have with
me right now, and anyway, whatever one thinks of their interpretations,
the *facts* they set forth do indicate that Nixon was at least keeping his
options open in a way that suggests that he did not regard the nomination
as an empty prize.)
https://groups.google.com/d/msg/soc.history.what-if/Ghzn1YByCDA/ww4p3unAnz8J
***
Anyway, to sum up my view: If Rockefeller had narrowly won the California primary, that *might* have prevented Goldwater from being nominated, though this is by no means certain. It is *very* unlikely, however, that it would be enough to get Rockefeller himself nominated. He was just too unpopular with everyone in the party except his outright supporters. To give him any chance at all, you have to go back at least as far as New Hampshire, where Lodge's write-in victory dealt Rockefeller a blow from which he never really recovered. (It was a blow to Goldwater, too, but one from which he could recover much more easily.)