Radicalism of this degree hasn't been majorly present in US elections in ages. From both the left and the right.
And I absolutely think there should be a level after which promises of change should be considered false advertizing. Democracy doesn't work like that. Change happens in degrees, not sharp turns. And the political systen should be responsible for educating people about this.
Radicalism is subjective. To some it won't be radical enough, to others it will be. Personally I feel like it's defeatist and unambitious sentiments not too dissimilar to your own, that have partially allowed the US to fall so far behind in the first place. So many people are so scared of the repercussions of asking for too much, that they continously end up settling for too little, or the same.
And of course change can happen in sharp turns, it's happened throughout history. Having that said, I don't think the changes proposed by Bernie are that dramatic in the first place, not when you consider the reality that were he in power, the actual policy changes brought to realisation would be paired back versions of those ambitions, such as is the case with any presidency. But you have to at least set the bar high in the first place.