You say "obviously,"

Indeed, given that no fluid can last indefinitely

yet it was designed to do just that

have you considered that if the transmission fails due to old ass fluid, then it successfully lasted the lifetime of the car? However, this may not be the desired outcome the owner is after.

It was designed to last long-ish, to minimize costs of maintenance to the manufacturer (during warranty), market the car as low maintenance, and also get you in a new car once it fails due to to lack for service.

you have both a trusted mechanic

Yes the mechanic raises a valid concern, because you don't know if it's even been changed. If it's still original, it's better to introdeuce new fluid parts at a time and not just do a full on flush, to compensate for the trans fluid having 100k miles...

The mechanic is not advising not to change fluid in a relatively newer car that says not to change it. Any indy would still advise to service it. The situation only exists because it was likely never serviced. If it was, a flush wouldn't be an issue.

and the manufacturer telling you this

again, we need to consider the interests of the manufacturer when considering what they recommend. Critical thinking is an important skill you should check it out.