To clarify, my perspective isn't just in regards to Sanders. Him, Warren, Buttigieg... some others I can't think of off the top of my head- they've all indicated positions that could be regarded as pulling further down the route towards autocracy.
The US's history with its flawed democracy should not be the basis to throw it away today. Yes, it has screwed up considerably in the past (even more-so than you indicate- think Alien and Sedition Acts), but that should not be the basis to say that "since it has never been truly a democracy, then it doesn't matter". The issue isn't that "we aren't losing it if we've never had it"- democracy isn't all-or-nothing. Personally, I view democracy in a different way. Imagine every person in a country as a point on a level surface. Their height above that surface is the electoral power that that individual holds. Imagine now the surface constructed by those points of people. The more democratic a government, the more level this surface is- each individual has roughly an equal electoral power. You can think of a measure of how "level" the system is by imagining putting that surface into a box, and then filling it with water until the entire thing is covered. Then pour out water until just the tip of the tallest peak shows. The more water that remains, the more unlevel your system.
Hence, when I speak of the sacrifice of democracy, I mean moving further towards the unlevel. Regardless of the distribution of electoral power throughout US history, it's still possible to move more towards level or more towards unlevel.
Yeah, I get that it's not a binary. We can get more or less democratic. I'm just putting things in perspective. How harmful to our democracy is the stuff we're talking about, really, given the scope of how bad it is already? What does it mean when we talk about sacrificing our democracy when the alternative is sacrificing a majority of people who are excluded from meaningful participation in that democracy?
Or... is it really sacrificing democracy if we use undemocratic means to increase democratic equality?
For example
A. Racial minorities are more heavily policed and more likely to be charged felonies for the same crimes than white people
B. Felonies make you ineligible to vote
C. The USA has a larger prison population than any other country on earth has had in the last 100 years, so clearly this is not nitpicking
Given this, aren't you bettering democracy by addressing those problems, even if you use undemocratic means to do it? Let's say you abuse executive power and pack the courts and so on to overhaul our criminal justice system so that it doesn't disenfranchise millions of people. Is the end result a country with better or worse democracy?
Btw, Bernie's criminal justice reform proposals are fucking killer. Achieving just 1/10 of it would be an incredible step. Achieving half would revolutionize the country in a very positive way.