Forget the metal particles. They are irrelevant. If they are large enough to be caught by the filter, they won't do any harm. If they are small enough to pass through, by definition they are below the critical size you need to worry about. Transmission fluids lose their additive package effectiveness more than anything else. Lubricity isn't the failure mode you need to worry about.

It isn't a material (fluid) science question. It is a system question. There is no debate that new fluid is "better" in an absolute sense. The two points of debate are (1) whether installing that high-detergent new fluid in an aged system that is contaminated from normal use will result in the detergents loosening and shifting the contaminants, thus causing a blockage in the valve body and subsequent failure, and (2) whether the fluid condition is the cause of transmission failures, early or otherwise, ie before the transmission would have had a non-fluid-related failure in any case. In the case of (2), the other approach is to put aside the money you would have spent on original Lifeguard fluid and save it for when a sensor or other item fails in the transmission. It may sound like it, but this approach isn't fatalistic. It is driven by the desire to reduce the total cost of ownership.

There isn't one correct answer.

Preventative maintenance is a good thing. But maintenance that isn't preventative is just sunk cost. After 12 years in that transmission, if it was shifting fine, I would leave it alone, as I don't think the mileage is as much of an issue as the time.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue