There’s nothing broadly wrong with building entirely new cities, but it’s incredibly difficult to do for a variety of reasons. Most attempts to do this fail. Expecting new cities to absorb all of the increase in population is unrealistic.
If you’re talking new suburbs of existing cities, well, that’s basically the only expansion we’ve done in many places. Some problems with that:
- It leads to more driving needed to get to the city (and thus more emissions, exactly what we need to avoid right now).
- If the implication is that you’re building outward because the inner suburbs are getting more expensive to live in (which will happen if they don’t densify at all and the population grows), then you’re basically forcing most kids to move far away from their parents/grandparents/etc. Some people won’t want to live near their family but many do, and being able to maintain those close family ties feels important.