Lifetime means the transmission lifetime, not your lifetime. And most bridges last longer than transmissions; I don't understand the bridge analogy, since we don't usually change the fluid in bridges.

I wouldn't say the new fluid shocks the transmission since transmissions don't really care about what fluid is in them, as long as it meets the spec. What the new fluid and all the detergents it contains does do is clean out all the build up in the transmission and flush it out. Since there is nowhere for the residue to go it tends to settle in the valve body, where it blocks small but vital control passages and orfices. Then, you can get shifting problems, caused not so much by the new fluid, as by the detergents in the new fluid.

If you change it frequently enough you will tend to avoid that particular problem (no guarantees, though). Then the real debate becomes whether your investment in frequent fluid changes has any effect on the time to failure of the transmission, and thus whether it was a good investment, or simply money down the drain. Changing the fluid only makes sense if not changing it contributes to failures.

What I hate is when people talk trash about tribology.

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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