most forms of capital - density, social ties, personal assets, personal skills - can be partially substituted. Absent density, you need some combination of skills, a lot more money, or personal connections.
What the implicit wealth of dense urbanism creates is a choice about whether to enmesh personally or not because cities offer impersonalized “collective human capital.”
This exactly. There are three ways we can get what we need: (1) do it ourselves; (2) exchange with others via bartering/favors; or (3) pay for it.* We all deploy some combination of those things, based on our circumstances.
When you're poor, you don't have much financial capital, so DIY and social capital are priorities. I've often wondered why many poor people don't move from economically-depressed areas to where the jobs are. It's in part because when you don't have financial capital, social capital becomes critical. You can't really uproot yourself to follow a job when you need your mom to watch your kids and your buddy down the street to keep your car going and your mom friend two streets over to provide hand-me-downs for the kid, you know? Social capital becomes a core value, because that's what makes your life work.
OTOH, people who have money can choose to deploy their financial capital to fill those voids. You don't have to put in the hard work to build social capital, or to learn to do something yourself, when you can solve your problem by throwing money at it.
And of course where you live also drives your choices. You need DIY skills and social capital if you live in a rural area where services aren't available and goods are nonexistent or very pricey.** OTOH, if you're in a city or suburb, goods and services are readily available, which means through supply and demand they're often cheaper. So even if you're not particularly wealthy, you can often choose to satisfy many of your needs buy paying for them. Of course, cities/'burbs also offer far more opportunities of things to spend money on, and less space/opportunity to DIY things to save money (you're not going to get many vegetable gardens when you're living in an apartment), and real estate can be very pricey given those same laws of supply and demand. So you also need more financial capital to live comfortably -- and salaries are often higher to compensate. But that also means you can tend to focus your own time and energy on increasing your own financial capital rather than on social capital or DIY skills, because that's what makes your life go 'round.
To my mind, Mustachianism actually values social capital far more than most of society. We are by and large a very wealthy nation by international standards. The vast majority of us have money that allows us to deploy financial capital for many things -- and as a result, we have grown a commercial society that teaches us to value all the things that money can buy us. Mustachianism reminds that we don't actually need to buy all that stuff, and that there is significant value in putting in the hard work to learn to DIY and to develop meaningful relationships that allow us to be useful to others and let them be useful to us in turn. IOW, just because you can solve most problems by throwing money at them doesn't mean you should.
I think the OP's question actually demonstrates this. A typical consumer, when faced with a move, is going to focus on amenities and what they will be able to afford to buy -- will I be able to afford a nice house, are there good schools or will we need to go private, are there shops and restaurants and movie theaters for entertainment, etc. It is actually very mustachian to say, you know, I've built a life that allows me to rely on social capital and DIY skills, and while I'd like to live in an area with a lower COL, I don't want to lose my social capital by doing so.
*There's also government programs that can provide things, but that's just a version of financial capital, just with the government paying via income from taxes, vs. you paying it directly yourself.
**We spend a fair bit of time in Taos, and the pricing is, umm, interesting -- you can get a $0.99 ice cube tray at Walmart, but if you need to replace a boiler, you'll get a quote for $25K (ask me how I know). We call it the "Taos tax."