Her statement elicited the support of other 2020 candidates. The same people who complain daily about Donald Trump violating norms are now openly advocating eliminating the Electoral College and packing the Supreme Court.
Democrats obviously want to beat Trump and win the presidency going forward. There are simpler, less far-reaching expedients than trying to dump the Electoral College, beginning with nominating a more appealing candidate than Hillary Clinton.
If Democrats could manage that, and win both the popular vote and an Electoral College majority, their concerns about the current system would suddenly evaporate.
The case against the Electoral College is, first, as Elizabeth Warren said, that it supposedly ensures that some votes don't matter: In heavily blue or red states, voters on the other side are effectively disenfranchised.