Well, one bad thing is that existing cities would need to be re-designed. It will take resources and decades of time. If we are patient, there is nothing bad there.
A potentially bad side effect: if planners take the easiest route and make the city ultra-dense and ultra-high, we get a vulnerable city that doesn't function if something is wrong with the infrastructure. People are, after all, known for taking easy routes (which may later prove hard for others).
Myself, I live in the countryside and don't like top-down planning at all, so I can't comment more. To me, the experience is typically: "can I build a road here? - no you can't", "can I make a thermal store? no you can't", I'm sure they will eventually tell me I'm producing solar energy the wrong way too... I know that things are different in cities because the threshold to disturbing others is tiny, but I don't think about it much - it is not for me.
I live in a 200k city and atm, city officials love to talk about density, claiming it was synonymous to sustainability. So currently, every available square meter is immediately dumped in concrete 30m2/p appartment building. The whole city is literally under construction but yet, traffic is not changing and even getting worse through all the construction. It's outrageous
IDK that it necessarily has to be that restricted, I live in a city where most things are pretty close enough to a 15 minute walk and it's just a small city that's largely suburban. Maybe on the outskirts of town it's more like a 30 minute walk, but that's still pretty reasonable.
For thousands of years humans have lived in "15 min villages" where almost everything has been nearby.
And most 15 min city definitions I've seen includes some sort of bike option.
I encourage everyone who lives in a city (or suburb) to look up routes from their home to groceries/pharmacies/schools/etc on google maps. I just did for myself and I'm already in a 15 minute city.
What's lacking is the infrastructure for people to safely get to these places without a car.