This does not really compute...

The brake system does not have any appreciable flow (at least it shouldn't in a properly working brake system, else you're going to have a really bad day)... When you flush the brake system, you're only removing the fluid that is available to be flowed, which is NOT the fluid that is locked inside the ABS actuating system...

Actuating the ABS system during normal use also does not create flow throughout the brake line system, it only circulates brake fluid through itself via the positive displacement pump & internal bypass system & actuators, it does not circulate any fluid to, or from the rest of the brake line system, nor new fluid from the reservoir... Actuating the ABS system only circulates brake fluid through itself, it does not get new fluid from the reservoir, thus changing only brake fluid, regardless of age, will not appreciably change out any of the fluid internal to the ABS unit, only the tiny amount of new fluid in the passthru channels will get mixed into the old fluid in the ABS unit...

The ONLY way to get the brake fluid exchanged internal to the ABS unit, is having new fluid flowing from the reservoir to the calipers, while the ABS pump is on & actuators are open, as this provides a constant new fluid stream at the locations inside the ABS pump that are opened & pumped thru the ABS unit, where the old fluid is then carried further down the brake lines to caliper & out of the bleed port...

Without the pump actuated, the entire ABS system is completely out of the loop during a brake flush, where the only fluid that is exchanged is the insignificant amount of fluid volume that is in the passthru channels...

If you examine the hydraulic diagrams for modern ABS / traction control units, you'll see that no fluid can be exchanged out of the ABS system, unless the ABS system is actuated while new fluid is being flowed thru it via a pressure bleeder. The ABS module is where there is the most likely chance for contamination of the fluid to occur...

Being very generous with the numbers: Assume that a very generous 10% of the fluid is exchanged out of the ABS unit if you do not actuate it during a flush.

1st flush, you'll have 90% of the factory fill that is 2 years old.

2nd flush, 81% factory fill fluid that is 4 years old, 9% that is 2 years old.

3rd flush, 73% that is 6 years old, 8.1% that is 4 years old, 9% that is 2 years old.

4th flush, 65% factory fluid that is 8 years old, 7.3% @ 6 years old, 8.1% @ 4 years old, 9% that is 2 years old...

Not flushing the ABS unit out, even at 2 year intervals, simply does not compute...