The Democrats have a problem that's more than just their internal structure. They are the more popular party. Their policies are generally more popular. But because of the structure of our democratic republic system, the urbanization of the country has been chipping away at their effective power for decades. Losing to Trump really is far less of a concern than figuring out how to fix their problems with mobilizing their efforts in the House and state level elections. Losing the Presidency sucks, but the President has a lot less power than people seem to think. As the Republicans have demonstrated controlling the House and having enough of the Senate is enough to grind anything the President wants to do to a halt, while having a lot of control over state level politics lets them push their agendas on a local level while the federal government is gridlocked (including measures to keep it that way).
There's also a moral/philosophic issue at hand. Do the Democrats continue to try to be the "high road" party, or decide to "fight like Republicans," and go full partisan nasty. It's hard to say what's the good choice.
The Democrats also have to hold their core base together. Changing their internal structures too drastically in response to a bad election risks just shifting which groups they're getting more votes from, and which groups they're losing votes they normally get.
Whether or not the Republicans can replace a faithless elector depends on the rules of each state, though a faithless elector that wanted to could sue over it if they wanted to. It just wouldn't really matter unless somehow that one elector was going to decide the final vote (which it's not at this point).