I've heard many people say that before (and while I admit, I have never changed mine in any of the Toyota's that I have ever owned) but that logic doesn't seem to make any sense.
Can changing your transmission fluid be the sole cause of your transmission failing?
I would bet money on it never having ever been the cause of any transmission failure unless you did something catastrophically wrong in the process.
Will it make your transmission fail sooner?
Possibly...?
If changing your transmission fluid causes your transmission to fail, then it was going to fail anyways.
Then I suppose you could say, why risk it and then why not get XXX,XXX thousand more miles out of it before it naturally fails?
I don't have an answer for that, it's up to you to take the risk of doing it and understand that eventually every transmission will inevitably fail.
With a 20+ year old vehicle, you should probably expect at some point to have a full transmission service done, which I'm sure is expensive, but is cheaper in the long running to do it on your own accord versus having it randomly pop up and leave you stranded somewhere.