How many people are using their phones for some other purpose for which they want their phones to never reboot? And what are they actually doing with their phones?
If you're using the iPhone as some type of IoT appliance, either time limit would be disruptive. But if you e.g. enable Guided Access, the phone will stay unlocked and so shouldn't reboot.
If you're using the iPhone as a phone, who the heck doesn't touch their phone in 24 hours? Maybe if you're on some phone-free camping trip and you just need the iPhone with you as an emergency backup—but in that case, I don't think Inactivity Reboot would be particularly disruptive.
Maybe Apple will lower the window over time?