Yeah, and memory is infinite, and all disks are free, and the computing fairies make all the processors super fast.
But back in the real world, startup time is a valid concern.
There is no problem with restarting nginx to update its config once a second, and it's even encouraged that you do so. Besides, crash-only software recovery is really the only morally sound development technique.
E.g. doing a rolling update across a large number of instances by restarting a service at a time can become a quick enough process to be viable in instances where you'd otherwise need lots of excess capacity to be able to cycle larger proportions of instances at the same time. Making full rolling updates "cheaper" both in time and resources also translates to making rapid updates a safer choice (e.g. if I can roll back a broken release in 5 minutes, it's far safer to push out a new release than if a rollback takes hours).