The mechanic might say that because the service manual says that, the service manual says that because it makes the initial servicing costs low, which appeals to new car buyers and the manufacturers marketing division, and the manufacturers and marketing division don't give a damn about the car once the warranty period is over.
But, the fluid does break down over time, typically it will last past the warranty period, which makes the manufacturers and marketing happy, but how much longer it lasts depends on alot of variables, is the car driven gently, has the car been driven hard, has the car been used for towing, and such, your miles may vary
In the end it is up to the owner, who should do their due diligence with some engineering based principles
In the end it is up to the owner, and their budget, how they want to roll the dice
For me, some servicing costs is better than the cost of needing a rebuilt the transmission because it lunched itself due to poor maintenance
A few hundred a year is better than the thousands it cost to rebuild a transmission
My advice is fine a "good" performance transmission joint and ask them, I would even throw in a transmission tune and cooler, that only costs and additional couple of hundred dollars each, if you have any power mods, a higher stall and/or give the car a hard time or tow
Opinions may vary