Why was Joe Biden's campaign always expected to collapse through 2019?

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Other Democratic primary candidates had the chance to go after Biden aggressively but didn't. If you look at how the primaries played out during 2019, you will notice that no decisive blows were landed on Biden during 2019; he could have been out had his opponents been more aggressive in going after him (e.g. Elizabeth Warren could have pointed out that her getting into politics was motivated by things that Biden had supported; instead, she pulled her punches and didn't do better than third place in any contest).

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Wishful thinking.

Once Harris dropped out the nomination was Biden's. Nobody was there to capitalize on the Black vote.

It's funny how IA/NH/NV were completely irrelevant. Black women choose the nominee.

That's not the whole story. Both Harris and Warren were top-tier candidates at some point, when they focused on economic populism. Their standings fell when they shifted their focus to social issues/identity politics. Had they stayed focused on economic populism, they might have been the beneficiaries instead of Biden when it came time for the establishment to coalesce around a single candidate.

In terms of going after other opponents, remember how Obama won the 2008 nomination - he did so by aggressively going after Hillary's track record, and never backing down. Not even my preferred candidate was anywhere near as aggressive in going after Biden as Obama was against Hillary in 2008 - so let that serve as a lesson for any future primary candidate wanting to come out on top.