Remember also that a certain percentage of voters will vote for the party regardless of who is there. Numbers in the 30% range are bare minimum.
I wouldn't say those people are unreachable, but you aren't going to reach them with a reasoned argument. They are emotionally attached to the Dem party and they are going to have to reach a point where something smacks them hard enough to get them to think about it.
My father was such a person and, based on my personal family history, I'm going to go out on a limb and say that such people absolutely ARE unreachable. In my father's case, he was a Republican, but over the years I saw him support some truly atrocious local and state candidates. "Why would you support that clown?" I'd ask.
"He's a Republican," was the answer. My father would have supported Attila the Hun if Attila had been a Republican.
And that's why I vowed never to join a political party. Our statewide 2A advocacy group is always pushing me to register with a party -- either party -- so I can vote in the primaries. Yeah, I get it. If both primaries nominate poor candidates, maybe I will have missed an opportunity. In the end, though, my objectivity is more important to me. If I register with either party, even for just one election cycle, I can never again write to an elected official or testify at a hearing and state that I am a life-long unaffiliated voter.