To be fair, it doesn't seem unreasonable to start with the assumption that something is unsafe and work to prove that it is safe. Seems safer ;)
If this was some technology where not having it would be a major impediment to society, then maybe it would be a different story. But 5G just doesn't seem that important to me.
If we were to do this, it would cause large numbers of techologies to never ever reach the public because people couldn't do the work to "prove" that something is safe. I would only recommend this policy towards things that we could reasonably expect to have catastrophic outcomes.
Like I said, the reason it seems reasonable in this case is because 5G really just doesn't seem like it's super important. The telcos just need a new thing to market, and it seems totally reasonable for us to expect them to ensure it's safe, especially give the ubiquity of cell phones and the fact that most people will probably eventually be upgraded to 5G without even needing to opt in.
If this was some technology where not having it would be a major impediment to society, then maybe it would be a different story. But 5G just doesn't seem that important to me.