In 20 years Biotcoin must be worth several million to be as safe as it is today. Or the fees or the transactions have to increase significantly. We know where the reward will be in 20 years' time. But we don't know how many transactions there will be then. I would guess not dramatically more than today.
20 years is a very long time from technology perspective. Just look at 20 years ago and the advances we have today compared to then (bitcoin didn't even exist back then). The advances are also getting faster, not to mention some of the cryptography algorithms will have been obsolete by then.