A healthy city needs a town centre. A commercial centre. "Downtown" is where you find banks, shops, offices, Municipal Hall, museums, professional services conveniently clustered. You go there to do business so you don't have to do business everywhere else. Beyond this commercial centre there needs to be a non-commercial fringe: the residential space.
Historically, towns began as commercial centres on trading routes, or places where transport routes intersected. For convenience and access to work, growing populations gradually settled near and around them, each family in their own house or cottage with its own food-producing garden and often a fence or hedge for privacy, and for peace and quiet.
The "fifteen minute city" has no minutes for peace, quiet and privacy. Commericalism is everywhere. There's no relief from business, from busy-ness and crowds -- the "madding crowd" which Thomas Hardy recommended getting far away from. There's no escape from what poet William Wordsworth called "getting and spending / laying waste our powers". He meant powers of reflection, of quiet unhurried thought. The old-fashioned residential zone beyond the Town Centre vouchsafed gardens, fruit trees, cats on fences, porches with a mailbox and a shelf for the sprinkler that kept the lawn alive on which the kids could play. The fifteen minute city means the opposite: compression and some supposed version of "convenience" ... but never fifteen minutes of solitude or silence.
How mentally healthy are people crowded together without solitude, silence, and space for reflection? There used to be an ideal of a Green Belt surrounding an urban centre, reached in stages of sub-urbia which gently declined into wooded space. Now we contemplate a city comprising only one continuous Grey Belt, in which "work, play, and business" are bundled together. This doesn't work for those who want a private house and garden some distance from noise, commercialization, sun-blocking high-rises, and jostling crowds.
