

Windows is in no way free. Every new Windows Laptop and PC comes with a license; when you pay for the PC part of that money goes directly to Microsoft.
Microsoft made upgrading to Windows 10 and 11 “free” for those on older hardware who already had paid for a license because they wanted to move people onto the latest versions and stop supporting the old versions. At the same time they’ve been harvesting and selling users data to make even more money.
They are not trying to “kill” Windows, they are trying to change it into a cloud based system too so that you do have to pay a subscription to use it. They want new PCs and Laptops to be essentially nothing more than thin terminals, using your hardware to support their cloud based system but not actually owning any of the software at all.
But they are less bothered about the absolute revenue Windows makes now, and more bothered about making it a walled garden they control and which up-sells you to all their other subscription services under Office, and Xbox.




How small is your smallest device? BTRFS doesn’t have a minimum size, but practically probably 50-100mb is just about doable before even just setting things up get complex. Having said that though, it’s copy-on-write and has overhead as a result, so may not function well below 1gb.
ZFS meanwhile really won’t work well below probably 8gb. It’s also copy-on-write but with a lot more overhead due to how it works. It really works best on big drives and filesystems.
If your old storage is in the mb range, then really neither will help you achieve what you want.
BTRFS and ZFS do offer the same benefits as NTFS with regard to compression and speeding up some slower devices (due to lowering the actual read/writes needed to achieve the same result as the data is compressed into a smaller space and decompressed rapidly by the PC in memory), but NTFS can go be used on much smaller disk sizes due to how it works. BTRFS and ZFS are designed and optimised with other benefits in mind. And NTFS compression isn’t well supported in Linux.