The thing is, the Dems kept moving gradually more to the Right over the last few decades, so any so called "extreme" candidate, if they were to get picked and they were to actually win, STILL wouldn't be able to implement any kind of wholesale extreme agenda.
For instance, if Bernie HAD won the election, there's no way he would have been able to implement any kind of actual "socialism". He'd have been lucky to get maybe one piece of signature legislation through, perhaps healthcare, maybe expanded college subsidies and that's about it.
He would have had to work with the "Conserv-A-Dem" Blue Dogs, Third Way, DLC et cetera, in order to get any traction on anything.
So the net result would have been he would have pushed the Democrat Party a little bit back past the center and slightly left and that's about it. And it's clear that's about the way it is now, too. The Democrats have their center, their center left and they have a small and very vocal progressive wing but in the end there isn't enough of a progressive majority to swing the party to an extreme position.
So, that's not "turning the country socialist", it's just a Democratic Party course correction a little ways back more towards what it used to be.
All that having been said, if we keep seeing a concerted push further and further to the Right, it might trigger the creation of a progressive majority as a response.
And still, given things are what they are, in the end we'd still only see a somewhat mild shift to the center left anyway. This country's economy hummed along remarkably well during the FDR New Deal and in the few decades that followed, so it's not like the sky is falling. Perhaps we should revisit some of that.
That's just my 2 cents but I did grow up during that era.