I think it's far more complex than that. Hillary - with all of warts - was as mainstream/centrist of a candidate as the Democrats could have put up, and she got beat. I'll grant you that virtually ANY other candidate could have beat Trump in 2016 - including a hard left populist like Bernie.
2020 is a different beast. Trump won less than 50% of the popular vote during the primaries in 2016. There was a heavy "never trump" sentiment. Fast forward to 2019 and Trump has support of ~90% of Republicans. The party apparatus is behind him. It's been years now since guys like Ted Cruz were questioning his temperament and calling him a narcissist.
So - is easier to beat Trump with small ideas that are acceptable on the center-left / center-right, while risking the motivation of the base? While Trump will have an EXTREMELY motivated base? Or are you better off with Big Ideas that fire up the base, risk abandoning the center, but directly undercuts Trump's populism?
The reason I don't underestimate a Bernie/Warren in the General is that economic populism in a General election in 2020 America is going to be far more popular than talking heads realize right now. Much of that populism undercuts Trump's populist appeal, and fleshes out a clear set of ideas against Trump's total vacuum of ideas. It will be really fascinating if it happens.