The likely -- not inevitable -- nominee

I don't have a preferred candidate for the Democratic nomination yet (it's way too early), but if I had to make a prediction today, I'd say that Biden will probably get it.  This isn't just based on his huge current lead in polls of Democrats.  One can point to many earlier instances where the clear front-runner so long before the actual primaries didn't get the nomination.  However, Electoral-Vote today made this point:

The Democrats' problem is that collectively they appeal to far more than half the voters, but individually they don't.  While Sanders is strong with blue-collar men, he is extremely weak with blacks.  Sen. Kamala Harris (D-CA) does great with blacks, but very poorly with blue-collar men.  Elizabeth Warren does well with college-educated suburban women, but is nowhere with blue-collar men or blacks.  Pete Buttigieg is the choice of many millennials, but is not so hot with Latinos.  The only candidate who seems to do reasonably well with all Democratic constituencies (and who is hated by none) is Biden.  In the end, this may make him the most acceptable compromise candidate.

This could help explain why, while polls show several Democrats beating Trump in key states, they almost always show Biden doing so by the largest margin (Quinnipiac even has him four points ahead of Trump in Texas).  Polls of the general election ask voters to envisage a candidate as already being the nominee and potentially becoming President.  Black voters' lack of enthusiasm for Sanders, or blue-collar voters' suspicion of Warren, etc., may well dampen support in such a scenario.  But every major group in our coalition would turn out for Biden.

This may also account for his higher perceived "electability", a word which it has recently become fashionable to disdain.  Some bloggers like to claim that this is just code for his being an old white guy as most Presidents have been.  But that hardly explains his broad appeal, especially after Obama won two elections with the highest popular-vote totals in US history.  More likely, "electability" means just what it sounds like it means -- that he's perceived as most likely to win against Trump because he would be more capable than any other candidate of unifying and rallying the variegated Democratic coalition after the primaries.

(Yes, there would be some ideological purists who would find Biden unacceptable for whatever reason.  But that would happen for any candidate who won the nomination -- the perception is just that there would be fewer of them for Biden than for the others.)

Electoral-Vote also observes that Biden is focusing his rhetorical attacks on Trump, not on his Democratic rivals, whereas many of those Democratic rivals have started sniping at each other.  Electoral-Vote interprets this is Biden thinking he already has the nomination "in the bag", but it strikes me as smart strategy for the primary fight.  Most mainstream, not-very-ideological Democratic voters are focused on getting rid of Trump and a candidate who seems focused on the same goal will naturally appeal to them.  By contrast, rivals for the nomination jabbing at each other over this or that seem small, petty, and ordinary -- politics-as-usual.  I noted a while back that Warren too was aiming her rhetorical fire mostly at Trump, and was rising in the polls at that time while most other candidates stagnated.  If she joins the circular firing squad, I think she'll lose that advantage.  The way for another candidate to win the nomination instead of Biden is to convince those mainstream Democrats that he or she would have the best chance of winning against Trump.

I have some concerns about Biden.  His preference for middle-of-the-road stances suggests that as President he might try for less than could be achieved; his announced climate-change policy in particular is inadequate.  His age is a concern, especially since Trump seems to be in the grip of advancing dementia which will make age more of an issue in November 2020 than it is now.  I'm not worried about his talk of appealing to Republicans for unity, which I think is just talk aimed at Democrats who don't realize such unity is impossible.  He could be felled by some scandal or gaffe, but that's true of any candidate.

But ultimately the most important question is who can beat Trump.  After 2016, no one should be so foolish as to assume that winning this election is inevitable, no matter what the polls say now.  The best nominee in the world can accomplish nothing if he or she fails to actually become President.  If the great mass of Democratic voters perceive Biden as most capable of defeating Trump, they'll probably hand him the nomination.