The ruling theory at her time was that organelles were produced locally, and eukaryotes were generally held to be the product of bacteria. She pointed out that there are a lot of similarities between organelles and bacteria, and when DNA analysis became available in the 1970s, then it was shown that they are genetically related. The support for the theory increased, and there were several attempts to prove it. In 1982 Michael W. Gray and W. Ford Doolittle evaluated earlier attempts to prove the theory in the article:
"Has the Endosymbiont Hypothesis Been Proven?"
Here they showed that earlier attempt were not full proofs. Instead they made a new analysis based on two different kinds of chloroplasts (plastids). One of the central conclusions in this 42 page article is:
Taken at face value, this means that cyanobacteria and red algal plastids diverged from each other more recently than either did from Euglena (or Lemna) plastids and thus either that cyanobacteria evolved from plastids or (more reasonably) that the most recent common ancestor of Porphyridium and Euglena (Lemna) plastids was not itself a plastid, but an oxygen-evolving photosynthetic procaryote. (my emphasis)
It is interesting to see that they mentioned the possibility that "cyanobacteria evolved from plastids". It does not seem that they gave this much weight, however. It is other reasons why they concluded that the endosymbiotic origin of at least one of the organelles was just nearly assured.
There are four possible sequences of the origin of eukaryotes, bacteria and organelles. Two of them have the bacteria as the original. In one of them bacteria became eukaryotes and these created organelles. That was the old theory. The other possibility is that bacteria were imported and became organelles. I illustrate them in this way:
B->E->O Traditional theory
B->(O and E) Endosymbiotic theory
But eukaryotes could also be the original. In that case there is also two possibilities: They could have been reduced to bacteria, which subsequently created organelles. That is the theory proposed by Patrick Forterre and supported by Anthony M. Poole and David Penny, the thermoreduction theory. Another possibility is the one that Michael W. Gray and W. Ford Doolittle only mentioned as a theoretical possibility: that the eukaryote created both organelles and bacteria. The two possibilities are illustrated here:
E->B->O Thermoreduction theory
E->(O and B) Organelle Escape Theory, OET
I have called the latter Organelle Escape Theory because some of the organelles that were created got a commuting role, and among them there were many that became autonomous bacteria. OET, the "Organelle Escape Theory", which describes this scenario. It explains not only the origin of organelles, but also the origin of bacteria. The possibility mentioned above, that cyanobacteria evolved from plastids is a direct consequence of OET. When they say that this possibility is less reasonable, it is not based on any reasoning at all. I think it is very reasonabe. The conclusion of Michael W. Gray and W. Ford Doolittle should therefore have been that either the endosymbiosis theory or the Organelle Escape Theory is nearly assured. But if OET is chosen, then we do not have to say "nearly".