Even if everything was encrypted (which is not the case) and that encryption could not be downgraded by a mitm (which is not the case - cf starttls), it can still be used to track whereabouts, or disrupt the connectivity or utilise any mitm zero day.
But a good question would be why 5g, and not every other computer chip in any computer and network equipment, which could be backdoored, and I doubt anyone verified the conformity of the chip to the blueprint sent to the factory.
5G is the line in the sand. It's basically the point of no return, we're fully and firmly in the "cyber era" or whatever we want to call it. Where (when) even the backup and disaster recovery plans are (at least partially) Internet dependent.
Going forward this kind of infrastructure will probably be more important than any other kind (because this will control all the others too).
Will this finally move the cost-benefit needle toward verifiable computing (open chips, open fabs, open *ware)? Who knows. But so far it seems the power brokers of the old era (eg. countries) are trying to maintain control.
But a good question would be why 5g, and not every other computer chip in any computer and network equipment, which could be backdoored, and I doubt anyone verified the conformity of the chip to the blueprint sent to the factory.