I don't buy it. There is a great temporal gap between 212 CE and the abandonment of the empire by citizens. Sixty -ninety years after Caracalla's edict, there were ample citizens motivated to fight for the empire. And it wasn't a matter of fighting for one's home or community. Aurelian's armies marched from one end of the empire to the other. The empire had adequate citizen support down to at least 361.
The economy was "restructured on a local basis" as Southern wrote, but the ideal of a united empire remained strong. That enabled Aurelian to achieve what he did.
That owed much to a labor shortage.
Even if that were true there was little if any discernible effect, in regards to the critical issue--strength of the empire--for a century at least, or a century and a half.
Several things jump out at me here. First, the Empire starts abandoning it's citizens by ceding territory to foreigners instead of arguing over which particular Roman is in charge of it. Subsequently Citizens abandon their efforts to reform the Empire. Cause -> Effect.
Secondly, Southern entirely misses the point of what actually happened (which Diocletian did as well.) Speaking as a successful entrepreneur and, a Combat Engineer (I have a bit of first hand experience with "Nation Building") two basic things worked together to dictate the economy between the end of the crisis and, the collapse of the WRE:
1. Coinage circulated that had more value regionally than it did state-wide.
During the crisis, localized coinage was minted and put into circulation. Following the crisis, that coinage retained it's value at a regional level but - due to politics - was
artificicially devalued in other regions within the State. As a consequence, Markets continued "looking inward" and regional/local economic exhange became inherently more profitable.
Diocletian's monetary reforms actually exacerbated this trend which would likely have worked itself out over time because the market would have funneled off the old currencies in foreign markets where politics didn't artificially devalue the coinage. Diocletian's policies represented a barrier to this.
2. A bigger Government footprint followed Diocletian's reforms.
Rome's biggest issue was effectively governing such a vast territory affected by so many unique issues. The Republic became the Principate largely because the Republican Government couldn't effectively govern an Empire. Thus, Rome became an Empire.
Following Diocletian's reforms, you had at minimum two different courts in the West - that of the Augustus and that of the Caesar. As a result, economies started to become more decentralized and therefore increasingly more localized. Worse, those courts could move at any point. That meant strong local economies
had to emerge in order to be able to "pivot" as needed, redirecting their output to a different market.
That is, in reality what we see because "strong economies" are not
necessarily those economies with
the greatest output per capita.
Having two courts presents the appearance of greater control but, it actually represents less control. It also conditions people to look for local solutions to local problems (which was part of what Diocletian actually intended.)
The Diocese also help maintain things by imposing more direct, rationalized control.
Again, Diocletian's reforms largely exacerbated this issue.
Getting back to this: the best educated and richest citizens were the senators yet by 400 CE or earlier they seemed to care as little about the empire as others.
Senators were no longer the only educated class. By now, The Clergy competed with them and, did so largely in parallel.
However, I would point put that by the end of the Social Wars of the Republic, Senators (Caesar and others) largely cared more about personal power than they did "the Republic" for similar reasons - the Republic
just wasn't working out.
Diocletian followed a similar set of needs, he had a vision and, needed personal power to implement it. Same deal, everyone had a vision, they needed personal power to implement it and, Diocletian had set the the rules in place that power was now most effectively derived locally rather than from special interest groups.