https://www.nature.com/scitable/topicpage/the-origin-of-mito...
https://www.nature.com/scitable/topicpage/the-origin-of-mito...
<https://en.wikipedia.org/wiki/Symbiogenesis>
Mitochondria were sufficiently established in general awareness to be a plot point of Madeleine L'Engle's A Wrinkle in Time, being a concept which fascinated the character of Charles Wallace Murray, a young prodigy, in the stories. Though I don't recall if symbiogenesis is specifically referenced. The first story in the series was published in 1962.
I suspect that strong confirmatory evidence came through awareness and analysis of mitochondrial DNA, a term which begins apppearing in Google Ngrams as of the 1960s. Full DNA analysis likely wouldn't have occurred until the 1990s or early aughts, when full genome sequencing became available, so the point might have been argued until then. It strikes me as a rather rear-guard objection at that point however.
(I'd tried a few times to make a clearer assessment of when the view became mainstream, with inconclusive results, though "proposed in the early 20th century, largely substantiated in the 1960s, and all but sealed by 2000" seems a fair statement.)
And yes, no one has questioned the existence of mitochondria for a good long while now.