I think there are three huge fundamental differences that occurred post-WWII. 1) Access to credit. Pre-WWII, if you owned a house, there is a decent chance it was paid for in cash, or maybe a 50% down payment / 5-year mortgage. Anything longer than that was almost unheard of. Through various regulation changes, financial institutions began to blur the lines between traditional bank, lender, and investment bank. This allowed millions of Americans the chance to buy a home via a 15-year or 30-year mortgage, who were previously stuck renting. The term "mortgage" in itself loosely translates to: 'til death! Simply put, both supply & demand for housing surged post WWII.
2) The elimination of red-lining tactics and even the build out of America's highway system greatly expanded the mobility of our society. This in turn increased the variety of housing that was built. Terms like inner-belt suburb, ex-urb, vacation home, timeshare, apartment complexes, condos, ski lodges, retirement communities, assisted living facilities, country clubs, etc. etc. became mainstream, while expanding the demand for housing outside a basic metropolitan core. Folks who were previously denied mortgages due to red-lining could all of a sudden take part in the "American dream" of home ownership.
3) Lastly, the impact of technology transformed our connectivity from local, to national, to global. In local terms, everything you needed to live was supplied more or less by your local town. This included employment, shopping, religion, civic organizations, entertainment, and public services. Fast forward to today, and a geographic diversion has occurred between locations that have an international presence, and those that do not. Themes such as a major airport, shipping port, high speed rail, wifi access, an educated workforce, and sadly even an Instagram following determine a location's demand for housing. Think about Amazon's HQ2 criteria for reference.
Of course there are many other factors regarding real estate such as immigration, demographics, social class, weather, etc. but I think they are minor.
As for the future of small towns, that is a tough question. On one hand, our society is more comfortable with working remotely, and the future of transportation should eventually improve accessibility for rural areas ie) the hyperloop. On the other hand, some small towns would benefit from consolidation just because demand is so low. But hey, there are plenty of folks who are happy with their peace and quiet.
The trend of millenials living in cities is certainly true, but another factor is that they may be priced out of suburbia. Living in an ex-urb or rural area that is cheap is simply too far from 21st century jobs, so they live in cities.
That's how I see it, feel free to debate!