2002 Solara Transmission Diagnosis Question

2002 Toyota Convertible Solara
71,100 miles
6 cylinder

I took my car into the dealership in which I bought it when the transmission started making noise and seemed to be shifting hard. The dealership "ran a diagnostic" on the car. Apparently, they drove it around a bit and they ended up removing the transmission pan to inspect the tranmission fluid (and charging extra for the effort) :evil:. They reported that the fluid was dirty but there were no metal shavings in the fluid to indicate a problem. They also found about $1400 of other things to be fixed on the car, likes brakes, belts and such as part of their $800 "service package" I managed to get out with a $250 tab for the diagnostic and replacement of the transmission fluid. :X

Two days later, I end up stranded on the highway as the transmission slips. I was going 55 and suddenly the car engine races to redline and the check engine light comes on. As I pulled over, the car acted like it was in neutral in any gear. :X :x :x :x

So I get the car back in to this dealership and they indicate to me that it needs a new transmission. No kidding. Fortunately, I have an extended warranty to cover it. However, I am extremely irate about the experience. When I complained to the service manager, he indicated that they had no idea the transmission was about to go and no way of telling that either. He indicated the standard procedure is to change the transmission fluid to see if additional problems would surface.

This was never explained to me but more importantly, is this correct? Basically he was telling me they cannot diagnosis transmission problems until they actually become major problems. Like me being stranded on the highway. Is this true? Or should have my whole ordeal been avoided? : :

Thanks for your help!