50 USB webcams in a single computer. Is that really possible?

There will never be a change in this design of an additional USB-2-Hub-Device inside USB-3 because it's the only way to provide reasonable downward compatibility. If you want to use USB-3 Speed you need to use USB-3 Webcams.

Let me give you a example for further explenation. Let's say:

Ok, we build a high speed railway. Because the railway is high speed we can put more slow trains onto it than on our slow railway. Hurray!

Technically it's the same for USB. Just because you could drive faster on the railway does not mean there is more space (bandwidth) for slow trains (USB2). The slow trains will drive slow on the fast track, making it impossible for the fast trains (USB3) to use the railway at full speed. To deal with this some engineers decided not to replace the slow railway with the faster one, but to build it right next to it. With this setup it is possible for the fast trains to drive high speed on the fast railway and the slow ones on the slow. Same for USB3 downward compatibility.

The railway analogy is flawed, although the problem remains for many hubs. It is possible to have 10 USB-2 devices uprated to USB-3 but that depends on the hub. It was the same for USB-1 devices in USB-2 hubs. Most hubs shortcut by providing only one controller which is one specific standard, but some provide separate controllers for each port so any can be any standard without affecting the others - these are more expensive. An intermediate solution is a hub that provides (for one commercial product) three USB-3 ports and four USB-4 ports. The standard specifically allows for running USB-3 and USB-2 devices to operate simultaneously, with the USB-3 devices having the advantage of the additional bandwidth.

I have been running two Logitech webcams on USB-2 for years at an appropriate rate (e.g. as I recall 15fps x 640x480), and using multiple cheap CPUs to process and ethernet to aggregate 6 cameras plus other USB devices. With USB-3 I can hopefully use a daisy chain of the above hubs to achieve this, with two USB-2 cameras (or other devices) per 3+4 hub, assuming I use a hub controller that converts between USB-3 and USB-2 rather than one that just acts as a separate USB-2 hub for the USB-2 devices.

USB3 cameras are available but expensive, and unnecessary - it is just necessary to uprate a USB2 camera to USB3 to be able to use them but I can't find a hub or converter that claims to do this (there were/are analogous devices for USB1 to USB2).