I'd be interested in opposing views, as there must be some, judging by the reaction, but the above seems very apparent to me - I don't see why people would disagree or consider it controversial. Sanders wants to tax high earners to fund the debt forgiveness, and even if that would be sufficient, those funds would not be available for other spending (e.g., medicare for all, or the next "should be free for all" program - "housing for all", etc.)