What you're arguing is based on the assumption that populism is and has always been used by demagogues, and as populism is rather more accurately described as a political campaign strategy, it only requires one example to tear down the always assumption. All I need to point out is Bernie Sanders and the results of his works makes it so that understanding the questionable aspect of our own society is not to be seen as taboo, and making healthcare more accessible as well as reducing wage gaps is not a bad thing. In fact, he alone enabled a faster rate of political shift to that direction and removed the taboo of those stances. Your stance should be that populism is questionable, rather than a firm always bad as that can be teared down by examples of people trying to raise the flaws of socio-economic structures.
One could argue anything as bad if it has been used by demagogues. Moderation is even a example. You could argue that moderates enables a form of negative peace by allowing structure of society to retain gaps between people, and arguably leads to increase of gaps by simply pushing asides forces that wants to address those gaps. Moderates could be argued to lead to Trumpism due to those observation.
At the end of the day, what matters is the impact of political strategies and whether they have been used to benefit others. It is how they're used that matters at the end of the day.