Huh? I could be wrong, being far from expert on these matters, but as far as I know all eucaryotes that lack mitchondria only produce ATP anaerobically, nor am I aware of any evidence that eucaryotes ever had the ability to produce ATP aerobically apart from mitochondria. And of course eucaryotes all retain the ability to produce ATP anaerobically. So what it is that you are saying "had to have dissappeared without a trace" is a complete mystery to me.
Yes, all eucaryotes can produce ATP through glycolysis, but this is very inefficient and is doubtful whether eucaryotes could have arisen without the more efficient energy systems provided by chloroplasts and mitochondria.
This still leaves the following problems for evolutionists totally without explanation:
1. the destruction of all freely existign chloroplasts and mitochondria. (the dog ate the homework)
2. the destruction of their ability to exist as self sufficient entities once they were supposedly taken over by eukaryotic organisms.
3. Why the mitochondria of all surviving organisms lack the same abilities of self-sufficient survival in all species.
4. Why there is no stepwise loss of abilities in even the most primitive species.
5. Why mitochondria and chloroplasts require products and proteins from the rest of the eukaryotic organism to perform its functions. (here it should be noted again that it is the same proteins and products in all species that are needed and that of course without all this in place the chloroplasts and mitochondria would have been a deleterious addition to the organism and thus selected against).
6. The problem of differing genetic codes in the mitochondria of different species is totally inexplicable in any evolutionary manner.
What all this shows again is that there is absolutely no evidence for the evolutionist assumption that mitochondria and chloroplasts were self-sufficient organisms at one time which became part of eucaryotic organisms through endosymbiosis. So my statement that this is just another factually empty assumption of evolutionists which is contradicted by strong observable facts is correct.