Hi Martin, Yes I agree that FT8 or any of the Weak signal modes do not imply using low power transmitters.
They are 'what it says on the tin' Weak signal modes. A station on 20Metres in New Zealand could be running a kiloWatt and be a very weak signal here in the UK -- FT8 would be appropriate in those circumstances.
However; The HF bands will always be there, QO100 will not be. Lets make sure it's amplifiers last as long as possible by using sensible power from our transmitters.