I'm not so sure that it's only possible to build affordable 3 bedroom homes _because those burbs have roads_. I agree that it's definitely the best path compared to my options in my locale, but there are other cities/countries where I _could_ afford a 3br in an urban environment and the roads are just as rare there.
If affordability were inherently positively tied to land-use-for-roads, I'd understand the argument. For me though, I think affordability is tied to location-demand and way way more people want to live in the urban environment that I'm priced out of.