I liked the big Texan. I was drafted while LBJ was president which was the best thing that happened to me. I went on to make a career out of the army. These what if events is impossible to say one way or the other with any certainty. Like you, I do believe if LBJ stayed in the race for his party's nomination, he would have received it. Challenges to sitting presidents don't succeed. What they succeed in doing is weakening the incumbent president.
Reagan did that to Ford in 1976 and Teddy Kennedy to Carter in 1980. Nixon held a substantial lead in the polls from August through October. It was LBJ's announced bombing halt on 31 Oct that narrowed the gap to where Humphrey almost pulled it out.
Now wouldn't that have been something if Humphrey had won. A president who received his party's nomination without even having entered a single primary. Of course back then, that was way before the modern primary system which begun in 1976. there were only 12-15 primaries and some of those were just beauty contests which no delegates were awarded to the winner.
Considering that old system gave us FDR, Truman, Eisenhower, JFK, LBJ, ranked 2nd, 6th, 8th, 10th and 13th on the rankings of presidents. Nixon was the last president chosen by the old system. There's always one. Now that's impressive, four president's in a row ranked in the top 10 and a fifth at number 13. I'm not counting Ford since he inherited his presidency from Nixon and lost to Carter in 1976. Carter was the first president under our modern system of primaries.
Modern system has given us ranked beginning with Carter, 27, 16, 22, 18 and 32, G.W. Bush. I omitted Obama and Trump, as one just left office and the other just entered. Historians say you can't get a fairly accurate rating of a president until they been out of office for 20 years. That give it time to see the long term effects of a president's policies had on the nation. We don't know that for Obama yet.