Only the retrospective knowledge that Trump, against every establishment anticipation, won the election lends the idea that Hillary Clinton ought to have refused to participate in the debates in a (to my mind, Quixotic) attempt to "de-legitimize" Trump even the remotest plausibility. For one thing, a refusal to debate Trump would have been portrayed by the Trump campaign as an instance of the very pusillanimity of the establishment cum political class that you say (assuming that I understand you correctly--the compressed nature of a "tweet" may be an obstacle to my understanding) is "the one thing Trump & his voters had right".
In any case, the key reason why the Clinton campaign assumed it would not be in their self-interest to attempt to "de-legitimize" Trump was their conviction--universally shared by the establishment punditocracy--that Trump was unelectable (where "electable" is somehow supposed to correlate with "approximation to left-liberalism" or, perhaps better, to "liberalism simpliciter"), and thus Hillary Clinton, however moribund her candidacy, was assured of victory.
It is proverbial that the failure to cognize one's position as vulnerable or defeasible sets one up for disappointment or even astonishment--and the astonishment of left-liberals and of others whose political perspectives are approximations or echoes, however remote, of left-liberalism or liberalism simpliciter, is inscribed in their political deeds to this very day, some six-odd months after the election.
The left-liberalism that is concretized in the Democratic Party has quite obviously become a species of fanaticism--and, while I'd be the last person on earth to deny the virtue (and maybe even the sheer necessity at all times) of fanaticism--it seems to me that this particular iteration of the fanatic spirit isn't born of an underlying vitality but rather of a decay of vitality.