No oil change removes all the old fluid. For example, the stuff residing in an oil cooler stays in the oil cooler. I've no doubt there are plenty of other areas in an engine which cannot be drained in the conventional way. So, any 'foreign' material added to the sump, such as a flush, will inevitably remain within the engine when the fresh oil is added. Flushes are meant to flush, not to run an engine on. I suspect the reality of engine flushing has more to do with marketing than it has about sound engineering.
Some old neglected engines, loaded with sludgy deposit may benefit from flushing but not many.
TC
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