I agree with your reasoning but disagree with your conclusions. Bernie was a watershed, his core voters would probably follow Bernie to hell if he said he had a chance of winning an election there, but the rest of the democratic voter base thinks he's too radical at best and a dangerous commie-lover at worst. The reason Biden clenched the nomination was because large parts of the democratic voter base thinks he's a good choice. The problem with those parts are that they are the boomer dems, the voters who aren't much seen in the public discourse (because they don't do internet savvy like the younger generations mainly) and prefers a traditional democratic candidate.
Biden is very much traditional, he's safe and non-threatening to the large swathes of the democratic party that were born in the 40's and 50's and will remain an important voter bloc for at least a decade until they start dying off and getting replaced. They don't make as much noise as younger voters but they are also not as fickle as younger voters. When the boomer voters get a candidate they like (and they like Biden waaaaay more then they ever liked Hillary) they will turn out to a man to support their Man. So you are right in your reasoning that the Dems need an inspiring candidate to mobilize the voters, but I'd argue that Biden is exactly that. He'll mobilize the Dem voters that would whiff out if Hillary, Bernie or Warren got the nomination, the voters who want a trustworthy, proven man of their own generation to lead the charge for traditional democratic values and policies. The other voters, the women and minorities, are sufficiently intimidated by Trump that they'll likely show up to vote in large numbers any way, because they have way more too lose with four more years of Trump then they do if a moderate, ineffectual Democrat wins.
I know it sounds kind of stupid, but Biden was probably the most tactical candidate possible for the Democratic party. He's popular enough with the boomers to ensure their votes and everyone else will fall in line because more Trump is such an atrocious possibility that they'll vote for any Democrat, even one as grey and stuffy as Biden. In essence, Biden is the Trump of democratic voters. Where Trump mobilized a lot of reactionary republicans that weren't being represented by "traditional" rep candidates, Biden is mobilizing a lot of usually passive, unheard dem voters because he's the boomer friendly choice.