That is not how safety margins work. Safety margins are meant to give a buffer for unforeseen circumstances, they are not a ticket to just cheat. With this logic... why stop at a 25A breaker (30 in the US)? Why not just plug a 20A device into a 100A breaker, or no breaker at all?
No safety margin can account for purposeful circumvention, which is what connecting a 15/20A outlet to a 30A circuit is.
You see no difference between 20 vs. 25 amps and 20 vs. 100?
I didn't say any difference was unacceptable, but a significant difference for 20/25 should not be accepted.
If the danger gets gradually worse for every 5 amps on the fuse, that's fine. Then the excess danger at 25 or 30 amps is only a tiny fraction of the excess danger at 100 amps. Good work.
If the danger has a sudden sharp increase at a certain amperage, then that amperage threshold needs to be further away than a mere 20/25 difference. Or even 20/30.
No safety margin can account for purposeful circumvention, which is what connecting a 15/20A outlet to a 30A circuit is.