An interesting, if somewhat simplistic analysis. I think that that the author is on the right track but perhaps reluctant to say what he believes straight out: Detroit is a case study for the effects of rampant liberalism. He completely leaves one critical factor out of the equation - politics - when attempting to diagnose what went wrong with Detroit - and indeed much of the rust-belt.
Decades of dhimmicrat rule is what spelled their doom. Piss-poor financial management and planning. Cockeyed social engineering. Parasitic unions. All sucking at the lifeblood that had made Detroit what it was.
So what of his alternate outcome examples of Boston, New York, and San Francisco? Is it really that they somehow blundered into the "sweet-spot" of taxation and (all other considerations ignored) that is how they prosper? I don't think so.
Have you ever been to a ghost town? The amateur archeologist in me is fascinated by them and I've been to several. One point of commonality in all of them - they're all in the middle of nowhere. At one time there was a spark that ignited and a community was formed. It ran its course and then withered, eventually dying off. The reasons why they lose their vitality are legion but in the end they all share a common characteristic: they all lost a compelling reason to remain.
There's another point of commonality in the three examples that the author listed - they're all port cities. Detroit's lifeblood is (was) manufacturing. When most of the manufacturing disappeared it lost its revenue base. Leftists, in their inimitable - and totally insane - style attempted to substitute taxation for the losses of revenue. No doubt their mamas never read them the parable about killing the golden goose.
Boston, New York, and San Francisco - and Seattle - have made many of the same idiotic decisions that killed Detroit. Fortunately (for them) they enjoy more diversification of industry than Detroit had and continue to be viable (for the moment). In other words they prosper in spite of themselves, not because of any forward thinking policies.
But the writing is on the wall. I mentioned seattle because that's (more or less) where I live and thus I am able to study it up close. The pinheaded socialists here are busily setting about the destruction of their community. They work feverishly to impose their Brobdingnagian utopia onto the rest of us. I can't tell you how many absolutely brain-dead decisions I've witnessed here - many of which would kill a less robust community. Lots of them represent major body-blows. But although we stumble, we don't fall because of that diversification of industry. Simply stated, we have more ways to generate revenue.
Naturally that's a leftist wet-dream and consequently we have wall~to~wall liberal loonies parasitically slavering for the chance to latch onto one and suck it dry...
...just like they did in Detroit.