You are fairly misinformed.
1. The PS3 could save data to usb, but could not save installed games to USB.
2. Both consoles (PS4 and xbone) come with fairly slow 2.5" laptop hard drives. This is why the new 1TB xbox comes with a hybrid drive, and why a lot of people replace their PS3/PS4 hard drives with hybrid or SSD drives. Many people are seeing DECREASED load times by moving to usb3 storage.
3. Any 3.5" drive can be converted to a USB 3.0 drive with a $20 enclosure. I don't know where you're getting "x10 more expensive". Often, due to how they bundle drives and decrease warranty period, USB drives are CHEAPER than the equivalent hard drive bought on its own.
4. You can plug an SSD into a usb3 enclosure, and you can even buy retail USB3 SSDs.
I agree Microsoft should let you replace the internal drive, but apart from the fact that having a usb drive sitting on top of the xbox is ugly, there's no actual downside.