Backups are a total black pattern where you either pay Apple for ever or the respective APIs are horribly broken.
It would require zero effort on Apple's side to integrate backup to other servers using the OS or other Apps. That means without silently stopping them or even worse slowing them down to kb/s once in the background.
You can backup your iPhone anytime you want to your own computer. iCould makes it pretty easy to do settings and config backups that will be included in their free tier.
There is no way Apple is going to let 3rd party could providers do backups directly. I doubt exposing the iPhone as a USB device over the internet with a VM running iTunes would work efficiently.
Assuming you've already paid Apple for the device and you don't want to make use of the 5GB free iCloud storage for backups, you could backup iPhone to iTunes on your laptop (encrypted) and then ensure your laptop was backed up locally also. This way you avoid paying Apple for ever.
So yes you also need to install itunes to backup on your computer, because why directly mount it as a usb drive without an apple app?
That would also need zero effort from apple, but I was talking about an online backup with since forever established protocols.
"I can't back it up completely" and "I can't back it up completely the way I want to" are two different arguments. It's fine if the second is the argument you actually want to make, just be clear you're making it.
