I think there's this false perception that downtown was EVER full of residents. It wasn't.

Detroit has ALWAYS been a bedroom city - multi-unit dwellings like Chicago or New York has in scale, were consistently prohibited by a racist city council, because in general, multi-units attracted all types of minorities. Most of Detroit outside of the grand neighborhoods was built in matchstick-style, poor construction. I know, I've torn down houses that are old but are functionally useless [[no one is buying a 700 square foot home nowadays, yes, 700 square feet, built in the 1920's).

Downtown has never had the population of those areas - most of those buildings, even those that are NOW residential, were all commercial and people commuted to, in general, from their single-family homes. Detroit has NEVER been a dense city like those areas - it's always been neighborhoods, duplexes, etc. Detroit's culture even in it's heyday was single-family homes - so it's no surprise we spread out.

Now, as to the reasons for moving - the freeways helped but were NOT the cause. The real reason is race-motivated policies of realtors and insurance companies. In short, if you got back from the war, the feds told you where you could build your new house -and you couldn't use it to rehab existing housing. Also, insurance at the time did not cover the replacement value, it covered the depreciated value - so, if you lost your fridge in a fire that was 8 years old, you would only get a percentage of the money required to buy a new fridge. They'd give you the money as if you sold that fridge used.

So, with that insurance being the case, people had additional incentive to leave quickly any area that was starting to fall apart - because they financially couldn't risk losing their stuff, as the insurance companies wouldn't cover them, whereas they could get full replacement value living in white areas.