I've seen it argued both ways and I've yet to see real evidence, especially considering many suburbs are themselves actually cities/towns, and that cities seem to fight tooth-and-nail to prevent suburbs from leaving.
Have you ... looked for evidence? I guess I always felt that it was self-evident that horizontal development costs way more in terms of roads, pipes, and wires, and at the same time raises almost nothing in terms of revenues. Residential-only development patterns never pay their own way. https://resources.environment.yale.edu/kotchen/pubs/COCS.pdf
Yes. And it’s not like you can tear up all the roads leading out of a large city (or just let them decay).