If you're a normal user, you don't care about whether the radio is active or not. You care about whether you can easily disconnect from crappy Bluetooth devices, or whether you can easily disconnect from broken cafe wi-fi to switch to cellular. The control center buttons provided by Apple do both of those things, without breaking Apple's ecosystem of devices.
Is this resulting control center behavior optimal? Sure is for Apple users. Everything that's broken disconnects and everything magic continues working. Is this optimal for tech nerds? Sure, if they're Apple users, because they quickly come to realize how useful "disconnect from this specific SSID today only, but continue using wi-fi when I go home" is. Is this optimal for people who want to aggressively control their radios at all times? Nope, sure isn't, you'll have to go to the Settings dialog and realize to do so the first time, presumably having missed out on endless Device Paranoia 101 courses that explain this.
Turning off Bluetooth and Wi-Fi is something that most users do not require of their device, and do not want of their device. Those that do want this for some sort of specific reason are the extremely rare exception, and just happen to be in slightly higher proportion here at HN relative to the rest of the world.
Yep. Many greybeards around here have fought the battle of "Why do I need a modem to access AOL? I got a disk in the mail, that should be good enough!"
It absolutely does not warn you that it is keeping some of those services going in the background.