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People used to think it was really rare -- until someone actually recreated it in the lab[0]. In this paper, the researchers created a strain of bacteria (prokaryote) that would only survive if engulfed by modified strain of yeast (eukaryote) that would depend on engulfing said bacteria.

And shockingly, it worked! The engineered bacteria replaced the natural mitochondria in the yeast. Wild.

But I guess to answer your question more directly, the cells that don't have the mitochondria would more likely die, because they don't have the fitness advantage.

[0] https://pubmed.ncbi.nlm.nih.gov/30373839/