As far as the amount of flex in a Lee Enfield action is concerned, I suspect it is a lot more than a ten thousandth. Set up a rifle with minimum headspace, neck size to substantially reduce fireforming, and cases will still have a shorter lifespan than if used in a front locking rifle.
Cases stretch in a Remington 788 rifle because of bolt compression, not receiver flex.
But rear locking actions do have characteristics not observed in front locking ones.
A Remington 700 receiver is heat treated, but I've not seen one that was "through hardened" or carburized. They can be drilled or machined without difficulty - for sight mounting holes, blueprinting, etc.
The AIA rifles obviously withstand industry standard or proofhouse testing. Prooftesting does not mean that a firearm will not fail at a later date.
Many owners report them to be perfectly satisfactory rifles.
Everyone's cup of tea? Obviously not. Certainly the product of a rather atypical company. Long term future? Who knows?