It's interesting how Armstrong got put down by the mid-60s, considering that many Black musicians in other genres had had little or nothing to say about race. Sam Cooke has been retrospectively re-engineered on the basis of "A Change Is Gonna Come," a one-off moment. Armstrong had his own one-off but very revealing, moment when he condemned Eisenhower's handling of Little Rock in 1958. Berry Gordy, Jr.'s roster of Motown/Tamla artists didn't speak out in the early 1960s. Gordy put out a spoken-word line of records, including King's speech at the March on Washington, but the Supremes and the Temptations? What Black artists appeared at the March? Marian Anderson and Mahalia Jackson. There were Black musicians in attendance, including Josephine Baker, Diahann Carroll, and Sammy Davis, Jr., but they didn't perform (why that was so is an interesting topic). Duke Ellington? His urbane polish let him do things that Armstrong couldn't--but Ellington's wonderful music about race evoked Black history and "Black Beauty," but was never overtly political.
Which makes a point about Armstrong's critical reception. There was a kind of condescension towards him because he was a working-class man of limited education, but he was too sophisticated to fit the beloved white stereotype of the unlettered Black blues musician. Much of the criticism of Armstrong came from non-Southerners, who expected Blacks to carry themselves differently--rather presumptuous, especially on the part of white critics such as Andrew Kopkind. In part because of his experience growing up in the South, Armstrong was understandably cautious about the more violent-sounding side of Black Power. And, as his Reverend Eatmore and "Lonesome Road" records indicate, he was suspicious of smooth-talking leaders who would leave ordinary people in the lurch.
In a way, Armstrong shared a lot with James Brown who finally did put Blackness at the center of popular music (rather than the periphery) in the later1960s ("Say It Loud, I'm Black and I'm Proud"). Both came out of the South, both believed in Black entrepreneurialism, both believed in "crossing over" musically. Who knows what Armstrong would have been like if he had been born later, but in some ways, his career would might have been like Brown's; Armstrong might have put race more towards the center of the music he created.