I think you have cause/effect mixed up a bit. Those cities are only "affordable" because people aren't there.

If a couple thousand people all simultaneously buy into an "affordable dense-ish urban area", that action would make that area un-affordable, and subsequently create all the same displacement/gentrification/nimby problems that follow. Your Detroit example is a good one, the dense-ish urban area now costs around $300-400k, because people already did what you've suggested. Unless your moving into lower-density mostly-suburban-ish areas in Detroit, your not really saving any money anymore.

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If your going to "Kickstarter" a city, let's make a brand new one. Find a bunch of empty land (greenfield or brownfield), and build a whole new dense city from scratch on it. This is easy/cheap to do compared to actual urban development (it's how all affordable suburban housing already starts), but just skip the suburb part and go straight from land to tall density, so there's no existing population to harm, no existing zoning issues to fight, no existing infrastructure problems to deal with. Go from nothing to 100% modern on day 1.

You could buy a bunch of land near say (picking a place at random) Hazelwood, Minnesota. Build your city with dense urban environment to start (enforce your own density rules, straight to 5+ story buildings). Fund an expension of light rail, and your just 30 minutes away from MSP airport by train, with direct flights daily to NYC, SFO, and SEA. And your already on I-35 freeway for freight.

There's such a shortage for affordable dense urban housing, that if you could provide affordable density with fast reliable transportation to existing cities (light rail to the closest major airport), your new city would likely fill up quickly, and help make these existing cities more affordable too.

I'm going to use your example of Detroit as my example here. One of the problems with getting people to move into less desirable cities like Detroit is that the less desirable areas are pretty nasty with poor resources, dirty streets and sidewalks, etc. Infrastructure is a joke in many of these neighborhoods, and the city government is still too obsessed with just particular areas for growth because that's where the tax money is. If someone would come up with a way to not just get people to come into the neighborhoods, but to actually clean them up and get city and state government on their side, that would be the real winner.