Most OSes don't let you send raw TCP segments without superuser privileges, so you can't just bring your own TCP congestion control algorithm in the userspace, unless you also wrap your custom TCP segments in UDP.
Drown other applications for your own benefit?
How would you swap out the TCP congestion control algorithm in an OS, or even hardware, you don't control?
> Drown other applications for your own benefit?
Fairness equivalent to classic TCP is a design goal of practically all alternative algorithms, so I'm not sure what you're implying.
It's entirely possible to improve responsivity without compromising on fairness, as e.g. BBR has shown.
On the contrary, introducing their own novel/mysterious/poorly implemented congestion control algorithms is not a thing I want userspace applications doing.
And if you worry about hostile applications on your own hardware, the OS is an excellent point to limit what they can do – including overwhelming your network interface.
One might even call this "congestion control"!
While you're still on your own host, your OS has complete visibility into which applications are currently intending to send data and can schedule them much more directly.