Hi Civical,
I'm not sure what mystifies you about the 85%?
This is slightly off topic for this thread, but you are happy with your outfit at 90%, but is this confidence because you have all the latest technology? What happens if any of it fails, do you loose control? I hope not. All the devices you mention are add-ons that hopefully expand the safety envelope rather than creating it.
Provided a vehicle has been serviced correctly, and any worn parts repaired etc, its age should not significantly change its towing ability.
What perhaps has changed is our perception of what a good or bad tow is. But regardless of age or technology, it is still the driver's responsibility to ensure that vehicle is used within its limits of ability.
As issues with stability are often (but not always) related to speed, then in general towing automatically reduces speed so it should keep the vehicle further within its safe operating envelope.
It is the nature of technological development that systems will improved, and in time the vehicle you drive now may be considered not as good as the best in some aspects of its handling in the future.
That's life.